Browse listings on major national vintage camper enthusiast sites. This will give you a good idea of price range and availability. Enthusiast sites with campers listed for sale include: Y Not Camper Restoration, Vintage Camper Trailers, Tin Can Tourists, Vintage Trailer Crazy.
Vintage Kingpins and Vintage Campers.
Look up local camper associations in your home state or region. While not all camper associations facilitate trailer sales, they represent a good resource for local vendors, vintage parts and places where vintage campers can park while traveling. A good listing of local and vintage-specific camper associations can be found at RV-Clubs.us.
Visit your area used RV dealerships to see if they have any vintage campers on their lots.