You can get a 32 quart (usually about 6 inches or so high, 27 or so inches long, and 16 or so inches wide.
Most people use these in rack systems, with heat tape, or flexwatt, as the heat source.
If you only have one or 2 snakes, you can get one that has the same dimensions as a 20 gallon long fish tank - long and high enough to put climbing branches in. If forget the exact dimension, but it is about 92 quarts or so.
Be sure to secure the lid, either with large binder clips (Office Max or Staples has these) or some other means. Drill plenty of ventilation holes as well.
YIf you don't want to use flex watt, you can always cut out a circle in the lid, attach hardware cloth (this can be melted into the lid with any fire source), and set your heat lamp on the hardware cloth.
Here is one I bought for $10 at WalMart: