Here's a really nice one sold at most WalMarts for around $35.00 hon. It has a thermostat. I have one almost identical to this one pictured that we use for a bit of auxillary heat in the snake room and it works really well. If it's a big garage though you might want to put up some temporary cheap walls just so you aren't trying to heat a huge empty space (heck hang heavy blankets just to trap as much heat as possible in the area you have the rats in, careful of course that nothing is actually in contact with the heater of course).

http://www.walmart.com/catalog/produ...Specifications