What you do is go to Big Apple Herp:
http://www.bigappleherp.com/bigappl...af043182802726424d361a05fb+EN/products/119105
and get yourself a piece of flexwatt heat tape. Go to walmart or whatever and get a *single pole dimmer switch*, like would be mounted on your wall, the kind with the rotary knob that would control how bright lights are. You want the kind that clicks on when you turn it, not the kind that comes on when you push it. You can get a metal junction box to mount the switch in if you like.
The heat tape will come with a cord which you splice the switch (basically a reostat) into. I had to get a longer cheap extension cord instead of the relatively short cord that came with the heat tape. If your snake cage is further than 1.5 feet from a wall outlet, you need a longer cord.
Heat tape comes with wiring diagram.
You'll also need electrical tape and maybe some wire nuts if you have them.
Once you get it all rigged up, put it under the cage, put a meat thermometer in the substrate where you can read it. Crank the dimmer up to about 3/4, give it 15-30 minutes to equalize temp, check thermometer. Adjust till you have the substrate about the temp you want (85 or so). Take a marker and make marks on dimmer switch knob and on the housing so that when you line them up you're spot on. Check temp from time to time to make sure nothing changes.
This is the cheap way to go, if you're not comfortable wiring it up, get an undertank mat, not a heat rock. Heating the substrate works much better than heating the air. But don't use a heating pad for humans. It's not designed to be on constantly, and the weight of the cage on it could cause problems eventually. You also dont have the fine control over temp.