There are various places online that sell both used and new tractor parts, such as Tractor-Part.com. This website allows its customers to enter the part number, make and model of the equipment to perform a database search and determine if the part is in stock.
A&I Products has a large database of tractor parts for customers to search. For those who own John Deere tractors, the John Deere website allows its customers to search its database to find the tractor part they are looking for. E-Replacement Parts has a large database of parts for Husqvarna tractors.
For those who don't want to purchase parts online, Fastrac has a large database, broken down by state, of individual salvage yards that carry used tractor parts. This database includes salvage yards that don't have websites.
Many tractor owners like to purchase tractor parts from a salvage yard because the costs are relatively low. Salvage yards let customers go out into their yards to find the pieces of equipment that have the parts; the customer must remove the part himself. This practice eliminates the need for stocking parts, which is why the cost of parts purchased in salvage yards are lower than those purchased in a store.