Yes you can. From a walkthrough, a person claimed that he received a Thunderstone from someone from Olivia City. It's a "she" that you have to battle and get her contact. Pretty easy though =]
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i have Pokemon soul-silver and i evolved my evee using a thunderstone
If you show Bill's dad a certain pokemon he will give you a evolution stone. He will totally give you a thunderstone. The last pokemon you show him is pichu, but after that you don't show him a pokemon forever.
Pikachu+Thunderstone=Raichu Eevee+Thunderstone=Jolteon These are the only 2 pokemon that evolve with the thunderstone in pokemon emerald
you can find one at the power plant
Talk to the worker guy on the top floor of the Route 9 Shopping Mall. He will give you a Thunderstone.