over the last couple weeks there have been SEVERAL threads regarding this very same topic, a quick search should answer all your questions.
Here's a quickie:
There are UTH (under tank heaters) that are made for reptiles. They are similar to a heating pad for humans i guess, but these are made to be on 24/7 whereas human ones are not. I got mine for around $20 i believe. Several ppl here use or have used human heating pads, but for $20 or so i like the added security of not worrying about a fire or malfunction in the pad.
Most of the "experts" (which i am not) here recommend having the temp controlled. this is easily done by plugging the UTH into a rheostat (Roughly $20) or a thermostat (roughly $100). this allows you to have more control of the temp instead of relying on the pad to simply "be on." Also it is much safer and by doing a search you will find more of the safety benefits of having the heat source controlled.
Any pet store sells UTH, and they are labeled as such. Just ask some1 at the store to point them out 4 you.
Hope that helps a little