If your finding thermostats at wal mart they are for your house, the kind you mount on a wall and controls your ac and heat these are different from what we use on a UTH. Thermostats are not cheap, any of them that are arent reliable and you shouldnt trust your animals with it.

www.reptilebasics.com thats about as cheap as you can find them anywhere

If money at the moment is a problem you can use a dimmer switch AKA a rheostat. They are around $15 and you can find them at lowes or home depot. There is a sticky at the top of the page on how to use them.