Petco used to sell it, for about TWICE the online price at $40. They discontinued it however. Try reptilesupply.com or reptiledirect.com, they usually have the ZooMed 500R on sale for around $22 plus shipping.
Or if you want something immediate,. you can use a lamp dimmer from Walmart or a hardware store, usually $9-$10. Not as good as a thermostat, but it will alow you to lower the temp on your UTH. You just have to check it often and adjust as needed.
Whatever type you use, you will also need a digital thermometer with a wired probe so you can get the temp right on the UTH.