It depends on where you live at, however you can always find cheap items or used items on www.ebay.com. You also might want to check www.Craigslist.org for used tractors in your area. If you want to go through a dealer, check your local yellow pages.
You can visit sites such as johndeere.com to purchase new tractors. Ebay is a great resource for used tractors. Tractorhouse.com is a great website used for buying and selling new and used farm tractors.
You can purchase used tractors the same places you would buy new tractors. Just like cars, they are often taken as a trade in on a newer model. You can also find used tractors in the want ad section of your local paper if you live in a rural area.
One may purchase used John Deere tractors from Auto Trader. They have a section in their publications that deals specifically with farm equipment and tractors.
One can purchase farm tractors new from a farm tractor dealer, such as John Deere, Massey Ferguson, etc. which could be found in a phone directory or online at yellowpages.com. One could also purchase a farm tractor from an individual by looking in the newspapers, online at website like www.craigslist.com. Also there are websites that sell new and used tractors such as tractorhouse.com. Tractorhouse.com has thousands of tractors.
There are several online stores that sell used tractors. Local tractor supply stores should also have some good used tractors.
You have to decide first whether or not you would like to purchase a new lawn tractor or a used one. For a new one, Sears is a good bet. For a used one, I would advise you to check on ebay.
An individual can purchase a used John Deere tractor in Iowa from Craigslist or Ebay. They always have a wide selection of used tractors, and some are at a great price.
Here's one shop right outside Des Moines where you can buy used tractors: http://www.vanderhaags.com/
One can purchase antique tractors at a farm in Kansas. There are many tractors in Kansas which means there are many antique tractors. Kansas is the perfect place to buy tractors, but it takes a lot of money to fly there.
Tractors are famous for engine problems and repairs can be costly. In the long run a new tractor with a warranty is worth the money.
A used Farmall 140 can be purchased from Farmall 140 Classifieds, Tractor Shed, Yesterday's Tractors, eBay, Kijiji, Antique Tractors and American Listed.
Some kinds of Case IH tractors that an individual could purchase are Axial-Flow, Farmall, Steiger, and Magnum tractors. All of these tractors come with a warranty.