Here's something that I used to do:
Take a sock, fill it with rice, knot the sock closed, and microwave it. It works like a heating pad.
Maybe you could also try wrapping a heating pad around the tank? Make sure there's someplace that is away from the heating pad, too, though, so that if a fish gets too hot it has somewhere to go.
I don't know if it is in any way bad for the fish, but that's what I used to do (the sock thing) to boost up my fishes' temperature in their tank when it was cold. I live in TX, so it doesn't get too bad, but anyways, that's what I used. Hope you find something out :)