Here are some links that might help:

http://www.machinefinder.com/

http://www.ironsearch.com/IRONSearch/content.aspx

http://www.fastline.com/Default.asp?bhcp=1

I wouldn't worry too much about buying a used tractor. Used is definitely the way to go if you don't want to spend a lot of money you don't have to. Most used tractors are decent machines and most dealers are reputable. Attending a farm auction might save you even more $$$, but you have to be a little more careful about studying the machine.