Whether you like it or not, the auction sites is where you buy grosgrain (and decent hats). If you get to New York, Paris or Tokyo, you can buy it at Mokuba, who still makes the right stuff. It's quite expensive, though ($20-25 per meter) - and they only will sell to you in person. If you find any vintage, it was probably bought from eBay or Etsy in the first place. If you buy it there yourself, it'll be cheaper.
+1
If you are looking for something other than blue: i.e. black, brown, khaki green, silver; I have some 1951 grosgrain I bought from a hatter's son, in a few different choices of widths . PM preferred, but don't know if you can do that.
"Faint hat never won fair lady."