Used tires can be found on Craigslist in the For Sale section under the auto parts link. Perform a search in the search bar to find the appropriate tire size.
Used tires can be purchased from dealers or owners. Dealers have a greater selection of used tires and offer additional services to go along with the tires, including installation and balancing. Owners may be willing to sell their tires for cheaper than a dealer, or even barter for tires. Also, a buyer can negotiate the tire price with an owner as compared to a dealer where the tire prices are generally set.