You should have some local dealers and obviously you can begin your search there.

Irby Tractors in Vincent, AL is a good place to start looking if you are wanting something newer. He can also find you something if you tell him what you wan - I believe he goes to auctions regularly.

Next you can look at what AG-PRO and TriGreen have on their websites.

Then you can look at Fastline or Tractor House and see what all the other smaller dealers have listed. You'll see all the smaller dealers with used tractors listed there and go to their websites and check out what else they might have.

That should put several hundred used tractors on your radar.