I love vintage quilts to and am always trying to reproduce quilts from the 1920s era. Try going on to Ebay and look up Effie's. she has tons of old fabrics. I don't know if she will have any really, large pieces, but it wouldn't hurt to send her a message and tell her what you are looking for. I've bought a lot of fabrics from her over the years and she is always very honest and reasonable with her prices.
Also, if you are looking for 60-80s prints then try the more contemporary/modern quilting stores. They have a lot of those mid-century modern prints, because they're so "in" right now. Honey Be Good and Cloud Nine come to mind. They both have websites.
Good luck!
~ Cindy