Herpstat and other temperature regulators.

[The Serpent Merchant]

There are a few cheaper options:

Hydrofarm: these are the cheapest thermostats commonly accepted. They aren't very accurate but are better than nothing: http://www.amazon.com/gp/aw/d/B000NZZG3S

Then there is the Ranco. These are much better than the hydrofarm but still not nearly as good as a Herpstat.

http://www.reptilebasics.com/ranco-etc-111000-pre-wired