Farm tractor tires can be an expensive purchase, particularly the larger ones that have traditionally been used on the rear wheels and are used for all four wheels on most modern tractors. Because of this, you might be tempted to buy used rather than new to save some money. Make sure you know what you're getting into before you make that choice.
Used tractor tires can be found at many tire shops and tractor dealers. They can be one of two types - used or retread. Used tires are those that have had a certain amount of use by someone else but still have the original tread (at least what's left of it). Retreads are tires that have been re-capped with new tread, using the original casing.
Each type of used tire has its advantages and disadvantages. Used tires with the original tread tend to have less structural problems, since nothing has been done to change it. But of course the amount of use they have had will be in question, since you want to be sure you will have a decent amount of life left in them.
Retreads, on the other hand, will typically have a longer tread life but because the tread has been re-capped you can run into structural issues with the tires. You want to make sure the dealer offers a warranty on these tires and inspect the casing and sidewalls for any obvious damage.
New tires are going to give you the best life and structural integrity, but you will pay for that obviously. New tires can be as much as double the cost of used, so consider carefully how much use they will get and whether the savings is worth the tradeoff.
If you're using your tractor on a working farm, where it is getting heavy use almost every day, your needs are going to be different than someone who uses it occasionally but not for any particularly heavy-duty work.
And how you use it will also have a bearing on the best tires. If you are doing a lot of pulling with the tractor, the tires are going to be carrying more of a load than just the tractor so you need to keep that in mind.