I find quite a bit and often for good prices, on eBay. Most of the merchants will tell you, if they are able, if the fabric is cotton, or feels like cotton or whatever. I don't pay as much attention to the definitions of "vintage" because I will also buy repros (like my beloved 30's) if I find them at a good price.
I have fabrics that are valuable (in a general market sense- desirable and therefor generally cost more) and less so, and the difference between the two is none- except for taste and trend. I will pay more for a barkcloth that has an atomic or machine age pattern, because they are more in demand right now. Also for large yardage, because if I want to make curtains or cover furniture, I need more. But smaller floral or landscape barkcloth- I am more likely to pass if it gets too high. There is plenty of it, and I don't want to be in a war.
I don't pay a whole lot of attention to "quilting" cotton, because that is a very new term. Our grandmothers (and mothers) used old clothing, old sheets, leftover yardage. There was no such thing as fabric made especially to quilt, except for maybe whole cloth.
I will say that it is much easier to sew a quilt top if all of the fabrics are similar in weight and texture. This does not stop me from using all kind of other things, but it is sometimes nice to have the smoothness of a consistent texture.
In the cloth I buy, I check for stains or small holes, I also am leery of musty odors, which sometimes come out, but sometimes indicate that the fabric is starting to break down. If a heavy cotton, like a barkcloth, feels cardboardy and very dusty (after washing) it is likely to be too brittle to hold up in a quilt or really anything.
I've had people come in my home and wonder when I'm going to get rid of the "original" drapes, which I made myself this year out of old unused stock. LOL. They suit my 1958 house and I like them. I don't get offended when others don't- I just do my own thing.