Is there a way to get more than one Lucky Egg?
Chansey can be sometimes be summoned by Elekid as an SOS Pokemon. Be warned that Lucky Eggs are a 5% chance of being held by a Chansey and even then, getting the Chansey to appear is already rare as it is.
According to serebii, Chansey and Happiny no longer drop lucky eggs. Unless you have used this method in this gen don't give advice that doesn't work.
I agree with the poni gauntlet. They give a high amount of exp and you can easily one shot them. If you run out of pp you can just go to the battle tree pc and put your pokemon in the box and then back. With the lucky egg and affection boost I get around 2,4k-3,2k exp for lvl 70+ mons every ~30seconds . The E4's pok?mon are a lot harder to beat because of good IV's and EV's and all the animations take too long, not only that they don't even give that much more exp. I think highest I got is around 6k?I've got on the moment four lucky eggs, I've received three from a trade. Some people were hopeless and places a Pok?mon with a lucky egg in the GTS. I took them directly when I saw them. I still need two for a perfect six, but I can better be happy that I've got four off them.
The place that I use to train is the Poni Gauntlet. That area before the battle tree, their are a lot off Pok?mon of high level. And the Pok?mons over their aren't annoying and easy to defeat.
Another option is the battle buffet, however it's only once a day, but it's worth it for the money.