Second-hand doors can be found at Habitat For Humanity Home Stores, yard sales, and antique dealers. Classified ad sites, such as Craigslist.org, also have used doors for sale.
Remodeling projects sometimes result in used doors for sale. Local classified ads can sometimes have listings for salvage building materials. Building suppliers often get returned merchandise that is considered used upon return but is in like-new condition. Charity thrift stores, such as Salvation Army and Goodwill, get donations of doors in resalable condition. Local antique dealers are a good source for vintage second-hand doors, although these doors are often priced higher than buying through a private individual.