I don't see why you shouldn't use an under tank heater. Unless you're confusing that with a heat rock. Heat rocks are NO-NO's with snakes....well any reptile really. They're prone to burns. But I'd say 99.9% of the successful BP keepers use UTH for their heat. They're crucial for providing belly heat which is needed to help digest food in the BP.

As for what to get..just get a human heat pad you'd find at any store like Walmart and such. Just make sure it has adjustable heat levels(low-med-high) and no auto-shut off.

What not to get...those reptile heat pads with the adhesive. They're known to get too hot, and the adhesive sucks because once you put it on the bottom of your tank, it's stuck there, leaving the tank useless in the future if you don't need to use the UTH on it.
Also what not to get...Heat rock, for what I've said earlier....the burn thing.

Hope I helped a bit.