Local used exterior doors can be found for sale on Craigslist.org. Alternatively, post a wanted ad on Craigslist for a used exterior door, and wait for a seller to contact you. Secondhand stores such as Habitat For Humanity sometimes have exterior doors for sale that are surplus items from construction sites or building supply stores. Ebay.com may also have local listings, and newspapers and local classifieds occasionally yield results.
When contractors renovate homes and businesses, they sometimes salvage building materials such as exterior doors, so check used building good stores or salvage yards for exterior doors for sale. Antique dealers can have vintage and hard-to-find exterior doors, but these are likely to be fairly expensive. Used doors made of solid metal or wood can often be restored if they are in rough condition. Instead of purchasing a new exterior door, consider refinishing an old one.
Surplus doors are another alternative. Many home improvement or building supply stores have a section of doors and other items that patrons have returned, and you can purchase these at a deep discount. Used doors can also be found at local yard sales or estate auctions. Sometimes, owners of dilapidated structures allow people to salvage items from a structure at little to no cost.