To find the cheapest used tires, search out nearby options on for-sale-by-owner websites such as Craigslist or Kijiji. You may also be able to find cheap tires on eBay or Amazon.com, or by visiting a local retailer such as Big O Tires, Used Tire Warehouse or Discount Tire.
Buying tires directly from an individual through for-sale-by-owner websites has the advantage of more flexible prices because people are often willing to negotiate. You may also be able to save quite a bit of money for the right tires if you're willing to drive to nearby cities and towns. Before buying from an individual seller, check out the prices at local discount tire stores to make sure you're really getting the best price.