Heaters For Betta Tanks

[Fish Sticks]

Right now I have a 1 to 1.5 gallon plastic tank for my betta (but hopefully he'll get a larger tank soon). The temperature is staying at about 75 degrees. Are there any heaters that are small enough for a tank that size?

 

[Samphire]

Rhubarb had a 20L tank and has a 25w heater. The girls have a 30L BiOrb and 50w heater. Custard has a 6l approx Hex and no heater, he seems very happy in there.