A rheostat will be a quick and inexpensive temporary fix, but a t-stat is really best for long term use.

You should be able to get a rheostat at Lowe's, Home Depot, Walmart, or quite a few other places. They might not call them rheostats though... look for lamp dimmers.

If you have not used the UTH's adhesive to stick it to the tank, you can put something inbetween it and the tank, even just a little more space. If you have stuck it on, supposedly they can be removed with the right thing to weaken the adhesive. Here is a sticky on how to do it.
http://ball-pythons.net/forums/showthread.php?t=66262