Hey!
Well, to answer your question (since no one has done it yet):
1. You should use RPG Maker XP if you want to make a "regular" pokemon fangame. There's something called "Pokemon Essentials" which is basically an engine developed by Maruno and many others to help you make your own pokémon game. Here it is :
https://pokemonessentials.wikia.com/wiki/Downloads
Now, you can make an equally awesome pokemon themed game using rpg maker 2003, like, for example, Super Pokémon Eevee Edition, which you can see here:
https://www.pokecommunity.com/showthread.php?t=224449
but it won't have the basics of Pokémon games. That just can't be done on RPGMK 2K3.
2. Resources are all over the internet, try searching for what you want on
www.deviantart.com, for example ("HGSS Tileset", etc). Remember to always credit the people who made the tiles or ripped them, if they want credit. Just read the descriptions and that should tell you.
Pokemon Essentials has a pack of tilesets included already though, they're 3rd gen, and you can start practicing mapping and eventing with them.
Here's the Pokemon Essentials wiki link in case you need it : (broken link removed)
And this sub forum is specially to asks questions about Pokemon essentials. So, if you don't know how to do something, or have any question, you can make a thread here and ask!
I hope it helps!