Rat snakes are an active snake that needs a lot more room than a python or boa of similar size...mice are your best bet for feeding in a captive environment, although in the wild they will eat just about any small rodent, bird, frogs, small lizards, etc. Mice are the most readily available so I wouldn't get too adventurous with the feeding options...
You're best best is to buy a captive bred rat snake...they are inexpensive as well and can be found at just about any herp show or through a breeder on the internet. I would avoid collecting one from the wild as they may never acclimate to captivity, could have parasites, not to mention that you don't want to adversely impact your local ecosystem......