There is a site called Missingfabrics.com that allows you to post for specific fabrics. I've been hanging out there connecting people with fabric pretty much since it began.
Resellers on Ebay, Etsy, and Amazon sell oop (out of print) and vintage fabrics. I usually go to Amazon first.
There are some other stores that are mostly in-person but some have web orders too. I'm sure other people will post some of their favorites.
It can be one of the hardest things, stepping away from what other people did, but ultimately a blue floral is a blue floral. You pick the scale, the tone you want and get something that is "close enough".
It's really sad though when we see the perfect fabric for our dream project and know the likelihood is very very low of every getting it.