I would think about hacking a 12V cordless drill motor and gearbox from a Harbor Freight or Homier 12V cordless drill. You can get the whole drill from either place for $10 and it is pretty much ready to use and you have a battery pack as well. Take a look at the second link in sanddrag's post.
http://www.harborfreight.com/cpi/cta...emnumber=47155
http://www.homier.com/detail.asp?dpt=1&cat=12&sku=02642
If you have the $ and want to just buy a mount instead of making one:
http://www.cncbotparts.com/handiworks_mount.htm
You could always just cut up the drill case and use it as a mount as well.
Search on Google and you will turn up a ton of links to Battlebot robots that use cheap drill motors and gearboxes.