Draft horses are sometimes available for sale on horse sale classified sites, such as Equine.com. There are also draft horse breeders who sell their foals and may have older horses for sale as well.
Classified sites often let the buyer search by breed, location and age to help narrow down the search. Draft horse associations are also a good place to look for horses for sale. These organizations can be regional clubs geared towards all draft horses or may be more focused on a specific breed. Some have lists of breeders who are members of the club or offer classified listings to their members who are selling draft horses or equipment.