Check with some of the larger real estate offices in your area. The company I'm with (Long & Foster, Burke, Virginia) has filing cabinets filled with the original sales folders and brochures from developers . . . . in this case, from Northern Virginia. Some stretch back 30 years or so. So if an agent is selling a property in a subdivision--even if the house was built 25 years ago--there's a good chance that he/she will be able to find the floor plan.
Hope that helps.