You can buy under tank heaters at most pet stores (~$10), or like I said you can use a human heating pad set on low.
There is also a type of heating element called flex-watt heat tape that you can use, however, you need to know a bit about electricity, because it's not fully assembled and you need to attach the cord, etc. You also need to have a thermostat rigged to it to keep it at the right temps, otherwise it will get too hot. This stuff is also usually only found through mail-order reptile supply stores.