You can buy an excellent Herpstat thermostat at the wheaton, IL reptile show/animal expo. There's a vendor there every show and he almost always has brand new herpstats. He's an authorized dealer so you get the full warranty and everything.

allanimalexpo.com is the show website.

Barring that, you can sometimes get a ZooMed 500R at Petco or PetSmart, but you will pay $40 or more plus tax when they can be bought online for about $22 plus shipping.

The vendor at the show selling Herpstats is GLK Herp
http://www.reptilenews.com/