I am aware you can get evolution stones at the bug catching contest, but is there an easier way to get a Thunderstone?
Non-repeatable - Sea Cottage (show Bill's grandfather a Pichu)
Repeatable - Route 38 (Occasional gift from Lass Dana if player has her phone number) - Bug-Catching Contest (post-National Dex, first place) - Pokeathlon Dome (2500 points, Wednesday/Thursday/Saturday)
Thunder Stone (Bulbapedia)
Look here.
You can get one in National park if you come in first in the bug catching contest. You can also get one on Route 38 (you will get a phone call) and on Route 25 if you show Bill's grandfather a pichu.
Look here also one more thing, if you win the bug catching contest you can get any stone. It isn't a 100% garuntee that it will be a thunderstone so that may not be the best idea.
I Hope this Helps :D