I have a Pichu and it hasn't evolved into a Pikachu yet because I haven't gotten his happiness all the way up. I'm planning on challenging the next gym with a Pikachu and the gym after that with a Riachu. Seems kind of unrealistic but I've done it before.
What are the best methods to obtain a Thunderstone two gym badges into the game? Can I even get a Thunderstone this early on?
Help please, thanks.

Go to the Underground and keep digging until you get one. That's the best way to get Stones, early or late game. It's random, but at least you can get them somewhat reliably.

I'll try it, I already got like four hundred skull fossils too.

For Sinnoh, underground is definitely the way to go. However if you want you can compete in Pokeathlons in Heartgold/Soulsilver and trade a Pokemon with a thunderstone over. Whatever you like doing best.

I did pokeatholons when I had Heart Gold and I'm saving up for it again so I guess I'll have to do both