Found another good way (much better than the Clamperl method actually) to earn a lot of money after the 8th gym. This one, however, requires more preparations (quite a bit more tbh). There are a couple of items you need: White Flute + Blue Flute (get them after collecting enough ashes before Fallabor city), Macho Brace (which can be found after defeating the family near Maville city), Acro Bike (switch with Mach Bike in Maville), an Everstone (found in Granite Cave hidden inside a rock), TM 46 Thief (found in Slateport city), TM 44 Rest (buy in Lilycove city), and Repels (any amount that is at least 1 is fine). Next, the steps:
1/ Dive to the underwater patches before Sootopolis city, and get yourself a Chinchou with Illuminate ability, then teach it Surf.
2/ Head over to Lilycove city, and use some random Walk Through Wall cheat to access the Contest Hall (this is not considered cheating imo, since originally this is a bug that should've been fixed), go to the Berry Blender in the upper right, and play with the old man, using Aspear Berry as the ingredient. Do this twice to get 2 PokeBlocks with Sour flavor.
3/ Go to the Safari Zone as you see the first grass patch, use the White Flute. Go to the PokeBlock Feeder and insert one of the Sour PokeBlock, which will make all Pokemon near that Feeder have a +Defense Nature. Use the AcroBike to hop on the same spot until you meet a Natu (which is quite common). Try to catch the Natus until you get one that has a Bold or Relaxed or Impish Nature, alongside the Synchronize ability, then put that one to the top of your team (Bold Nature is most recommended). What this will do is make wild Pokemon have a 50% chance of having the same Nature as the Synchronizer on top.
4/ Go up from where you entered, you'll see there is an area than can only be accessed using the AcroBike. Hop over the ledges to pass it. Go to the bottom left of the area where there is another Feeder, insert the second Sour PokeBlock. Now wild Pokemon near that Feeder will have a 62.5% chance of having Natu's nature.
5/ Use that AcroBike to hop in one place again until you catch a Female Dunsparce with the same Nature as the Natu, also catch a male Dunsparce (any Nature is fine). After doing these, retire from the Safari Zone (continue if you like). During this time, you can use the White Flute again just in case.
6/ Fly to Lavaridge town, exit down the jagged passes, go into the cave near the cable station and catch yourself a Slugma (anything is fine as long as it's a Slugma).
7/ Fly to Maville city, withdraw the Slugma, female + male Dunsparce, deposit all others, give the female Dunsparce an Everstone, and give the male + female Dunsparce to the Daycare. Exchange your Acro Bike for a Mach Bike for speed.
8/ Ride your Bicycle around as you gather Eggs from the old man in front of the Daycare. Slugma's ability Magma Armor/Flame Body will halve the egg cycles, making the eggs hatch faster, while the Everstone will make the eggs have a 50% chance of inheriting the Nature of their mother.
9/ Just ride around collecting and hatching eggs until you have 5 Dunsparces with Run Away ability, alongside the Nature of your Natu (which is the same as the Mother). Also, while you're waiting for the Eggs to hatch, travel down to New Maville and catch yourself a Magnemite with Magnet Pull ability. After you're all done, swap back the Mach Bike for the Acro Bike.
10/ Fly to entrance of Victory Road (you can also Surf and Waterfall there if you haven't arrived yet), deposit all your Pokemons, withdraw the Magnemite and other high leveled Pokemons, leaving one spot on your team.
11/ Withdraw one of the Hatched Dunsparces (Lv 1), give it a Macho Brace, make the Magnemite your first Pokemon and the Dunsparce your second. Go inside Victory Road (just staying near the exit is enough).
12/ Use your Acro Bike to hop inside the cave as you meet Pokemons, couple it with White Flute to accelerate the process. Battle them until your Magnemite faint (which should be easy). Now your Dunsparce should be the next option to send out.
13/ Keep hopping around as you meet more Pokemons. Magnemite's ability Magnet Pull will give you a 50% chance to encounter a Steel Type, which should be Lairon, Aron, or Durant, all of them giving Defense EVs. Only battle them if the wild Pokemon is one of those 3, or a Loudred/ Hariyama/Makuhita, otherwise escape (Dunsparce's Run Away ability will ensure an escape, so don't worry). Each battle, your Dunsparce will be sent out first, then switch it with a higher level Pokemon and KO the opponent, sharing the EXPs and 2x the EVs received.
14/ Keep battling until you're about to reach level 25 (meaning that you're at the end of Lv 24), then exit Victory Road and enter the Pokemon Center. Deposit your Lv 24 Dunsparce, recover your team's health.
15/ Repeat steps 11-15 for the remaining Lv 1 Dunsparces. Do NOT use Rare Candies for this, there's a reason why we're EV training those Dunsparces (albeit half-heartedly). If you want to prolong the leveling time in order to gather more EVs, you can give your best Pokemon the EXP Share, reducing the amount of EXP that Dunsparce receive by at least 50% (which means it will be 2x longer).
16/ Fly to Rustoboro city, deposit all your Pokemon, withdraw only the Chinchou, and 5 Lv 24 Dunsparces. Teach the Dunsparces Thief, and Rest. After you're all set, travel up Rustoboro city and Surf over the beach. Go forth for a bit and you're now inside Meteor Falls.
Earning money steps:
1/ Put your Chinchou on top of the team. Battle random pokemons until your Chinchou faints. Your team will now have 5 Lv 24 Dunsparces left.
2/ Use one Repel, then use the White Flute, also make sure that the Chinchou with Illuminate is on top of your team (even if it's fainted). What these will do, is to make all Pokemons with Lv less than 24 to not appear. Inside Meteor Falls, only Solrock, Lunatone, rare Golbat, rare Wooper have level higher or equal to 24, meaning that only they can appear. Out of those 4, Solrocks and Lunatones take up about 75% of them. The fainted Illuminate Chinchou will help you meet Pokemon more frequently, the same as the White Flute.
3/ Hop in one place with your Acro Bike. Every time you encounter a Solrock/Lunatone, you will battle it. Normally, if your HP is at least 45 or higher, you should always use Thief, which will steal the wild Pokemon's items. Solrock has a 100% chance of holding Sun Stone, while Lunatone has a 100% chance of having Moon Stone, both of these can be sold for 4500$ each. If your HP is less than 45, proceed to use Rest, then immediately escape. Use the Blue Flute to awaken your sleeping Pokemon after that.
4/ Sometimes, Lunatone will use Hypnosis. If so, then immediately switch to another Dunsparce and proceed as step 3 again. However, most of the times, both Solrock and Lunatone will just use Rock Polish/ Rock Throw/ Psywave.
This method requires extreme amount of preparations, sometimes too much for a person to deal with. However, after the preparations are complete, this is the fastest way to earn money. For example, after exactly half an hour, i managed to steal 56 Sun Stones, and 47 Moon Stones, giving me an amount of 463500$, which is 3.3 times more than what the Clamperl farming method gave me, an amount that would require you to travel half of Hoenn's map to achieve. This is even more useful later in the game when you want to buy more EVs boosting medicine for your team.
*Note: Furret is an option over Dunsparce, with higher Speed than Solrock/Lunatone, meaning that you won't have to be afraid of Lunatone's Hypnosis. However, Furret has a much lower base HP (85 compared to 100 of Dunsparce, meaning that it wouldn't do well against Psywave which deals constant damage), lower base Defense/Sp.Defense (64/55 compared to 70/65 from Dunsparce). Also, Dunsparce with 0 EVs Atk with +Defense Nature will never be able to KO a Solrock/Lunatone with thief, even if there's a critical hit, meaning that you won't have to worry about an accidental KO happening and leveling up your Dunsparces to Lv 25, meaning no more Solrock/Lunatone to appear (they're both at 24). Blue Flute is the bane of this strategy, awakening a Pokemon right after Rest, while being able to be used over and over. Illuminate and White Flute speeds up the money earning speed by 3 times. Run Away ability is needed in that it lets you escape an unwanted battle, especially against a Golbat, preventing unnecessary damage from Solrock's Fire Spin/ escape failure.