Can you get a Thunderstone in Pokemon SoulSilver?

No. The only way to get the stones, (thunder, fire, leaf, water,) is by beating the Elite four and goind into Kanto reigon. But that doesn't mean Eevee can't evolve at all. It can still evolve into Espeon and Umbreon! XD But to get Espeon, train it in the daytime until it is level 30. Then it will evolve into Espeon, and the same for Umbreon except you have to train it to level 30 in the nighttime.