Hey beta guy, recently I have seen these mini heater sets for betta bowls at petco. I think there made by Tetra Finn, but in some store they just hang them in the side of the betta rackets. Its a dark round circle heater that has a diameter of about 3 1/2 in. Its about 20 bucks, go to your petsmart and see if they have any. ;)
I think u can get them at Wal-mart for $10.00.