Only for very short-term use. Human heat-pads are not designed to stay on all the time, and pull a lot of electrical current. Leaving it on runs the risk of fire. Commercial UTH's use much less wattage, and are designed to stay on constantly.
I get mine on eBay, too. 50-60 gallon Zoo Med for 21.99 each, plus 8.99 shipping for several combined - the seller is shoppingdeity. At PetsMart and Petco they're $45 each. She's a sweetheart, too.