You can also buy a rheostat (lamp dimmer). Thermostats typically run $100+, so if you're on a budget you can pick up a $5 rheostat at the hardware store. They are just rotary dimmer switches. You will have to wire it to your piece of flexwatt (or heating pad) All you need is a soldering gun and some solder (found at the hardware store also.. inexpensive). I as well as others here can explain how to do it. It is very simple! Almost all of my snakes are on a setup like that, and I'm satisfied with it.

You can also have Matt (justcage) wire up one for you with a piece of flexwatt.