I have heard of people using small repitle heat pads
underneath smaller tanks to heat the water.
Failing that the lowest wattage heater I've seen is 25w, but I think that would be way too much power and take up too much foom for such a small tank.
if you are still looking for a heater..... Walmart sells a 7.5 watt for 2-5 gallons. I have one in my 2.5 gallon and it keeps it at 79F for my betta. Drawback is you cant adjust the temp.