If you are interested in a certain brand (say Airstream), you might want to start with a specialty group/forum for those units especially. You could also call around to the largest RV rental places in your area, and see what is available or where they can direct you. Vintage trailer rentals will probably be difficult to find (similar to the chances of Enterprise or Avis renting classic cars... just not in their business model), but I'm sure there is someone out there someplace that has this kind of service available.