I've not seen a thermostat in a pet store and have heard the same from more than a few posters. I think the chances are that they won't find one, in which case, they may as well save the money and get the Flexwatt, seeing as they won't be able to use whatever heater they DO find at the pet store until the thermostat arrives.
That is assuming they can't find a thermostat. In that case, the Reptitemp 500R is the cheapest and will get the job done. It is bottom-line. I think a "good" one will probably run around $100. I use a Reptitemp and it's a ***** to adjust and fine-tune.
The heater should help. A layer of newspaper or something will keep the snake off the glass, which absorbs heat much faster.
Edit: Actually, Monica makes a good point about the heater making the tank too hot in it's struggle to heat the whole room. Use with caution?