Where in FireRed can I find an Thunder Stone?
You can purchase the elemental stones from level 4 in the Celadon Department store. Prices vary, but all of them sit around the 2,000 PokeDollar mark.
Or you can find them at the powerplant.
Well, you can buy it at the Celadon department store, or you can find it at the power plant, which is also where Zapdos should be. Good luck :)