There are so many beautiful things out there. Certainly check with local museums, especially if you can match anything with local history/pictures in the paper/whatever. Provenance can make the difference, but in general the availability of such things is large and the demand is small.
I'll try to find some of the websites I used to visit when I was collecting vintage fabrics, my market was always more for the conservators of fabric rather than the home sewer who just wanted a fat quarter but I was mostly active in that about 20 years ago and most of my old links are no longer valid.