I know you asked for a tank, but if you wanted to, you could use a tub. I don't know if anyone told you that, but I'm telling you now. Just go to Target or Wal-Mart, get a Sterilite or Rubbermaid tub (size depends on what size snake; if you're getting a baby, the 16 qt. will work as long as you can fit two hides, a waterbowl, and a thermometer), and either drill or solder some holes in. I'd recommend a soldering iron, because if you drill the holes, they leave rough edges and sometimes crack the tub. There's a $10 soldering iron at Wal-Mart in the hardware section.

If you want a tank and know how to set it up to keep the humidity and temperatures right, go ahead; I'm just telling you there's another way besides paying for an expensive tank to house your ball python. Do whatever works best for you and your snake.