Under Tank Heaters (UTH's) can be purchased online from any of the following:
reptilesupply.com - Reptile Supplies.
Reptile Supplies, Dog, Cat, Small Pet Supply & Aquarium Products
- The Bean Farm

At least a couple of those same sites also sell Flexwatt Heat Tape; another option that is often cheaper and more economical but requires some assembly (generally). I didn't compare prices or specifics on those websites, just pulled em off the top of my head, there are other options out there! = )