Used outdoor boat motors are available from a number of retailers, including ARG Marine, SmallOutBoards.com, BoatMotors.com and BoatTrader.com. When all else fails, find rarer outboard boat motors on eBay.
Used outboard boat motors are available for all of the major brands, including Honda, Yamaha, Mercury, Johnson and Evinrude. All of the above sites deal in motor parts both new and used, as well, so they remain effective resources should something go awry with a new motor. Warranties can vary significantly from vendor to vendor and even item to item, so be sure to know exactly what is being purchased. EBay's selection is constantly in flux, so there is no guarantee that used outboard boat motors are in stock any particular day.