This is a good source, http://www.robotmarketplace.com/products/battlekit_drive_modules.html
More motors here: http://www.robotmarketplace.com/products/motors_geared.html
You will need something in the 2-3" motor diameter range, along the lines of something that is twice as powerful as a battery drill motor (10-20 amps). More in line with higher performance motorized skateboards. http://www.zboardshop.com/
Remember that everything on Craigslist is roughly %40 off the original price.
For a robot of 20lbs or so you will need to address the high current load on the motors by making sure your speed control is hearty. You can experiment with cheaper controllers and just upgrade once they burn out. The supper duty controllers are either wheel chair controllers or some of these. http://www.robotmarketplace.com/products/speed_controllers_main.html