How to heat my beta fish's water?

I've had my beta for about 7 months. He lives in a small kiddie tank that I would guess is a little under a gallon. He keeps getting sick, first with pop eye, then with ick. I keep reading that warmer water helps with their immune system, and I want to be able to warm up his water. How can I do this without buying a new tank or expensive contraption? My parents have a vendetta against my fish because they didn't want me to get it in the first place (he was at the 99� store in a cup of dirty water, and the only fish there, I felt so bad for him), so they're sick of buying fish medicine and won't let me get him a better tank that's temperature-controlled.