Most of what I do nowadays is donation quilting and I mostly buy fabric at the thrift stores for $1-2/yard. It helps that I live in a relatively affluent part of the country and I am lucky enough to find wonderful bits of yardage, along with a lot of rather strange prints that I am able to use for backing. No, it doesn't work if you are looking for a specific amount of yardage for a specific piece, you have to be able to pick out the good stuff, buy it no matter how large and collect it until you have enough for a project. Around here all the thrift stores have sales on the weekend, basically they get more stuff than they can keep so items are only in the store for a month, as they come in they are given color coded tags, after a month everything is super marked down and then shipped out of the stores. People like me go weekly, we typically have a route -- lol you get to know your competition. If there is something I really want, I buy it "full" price, but I try to wait and see if it will go on tag.
I suggest looking/placing an ad on Craigs List or similar places looking for donations, again as Lena said specifying what you want. There is a "Wanted" category which is the proper place, there is also the "Arts + Crafts" category. I currently have half my couch covered with fabric donations that are NOT quilting suitable for a specific recycle group but they are closed due to Covid. The ladies in my group are often contacted with handfuls/closets of fabric and we go through them separating out the usable from the non, the worthwhile and the not. Some stuff we are given are just trash ultimately, but we are making informed decisions. If you go the ad route you might also be contacted about "stuff" like, we are cleaning out grandma's house and there is a closet of sewing stuff. That might involve machines in various condition or other things to deal with or know in advance you don't want the contents of old sewing kits.
When I was stash reducing a church group got several bags of yardage, scraps and UFOs because of an ad on Craig's List. Girl Scouts got a bag. A local theater group needed ties... I had ties from a quilt project. All of my craft supplies went to an elementary art teacher (budgets have been slashed). During lock down, a "curb alert" (free stuff sitting outside) was put out for quite a bit of fabric suitable for quilting or mask making (hundreds of yards it looked like) -- not by me and I didn't get any because I had plenty but I did look at the ad and thought about going over.
I've greatly reduced my stash, largely by giving fabrics to individuals and groups and most of those are local and I've found on Craig's List. I've achieved most of my reduction goals at this point and don't have any I can offer your group. I have one last box going to a hospice group of nice but dated 80s prints that don't work so well for the kids charities I'm usually doing projects but work well together. When I send fabric, I typically send it for free at my cost. I can tell you postage adds up, I use the flat rate boxes and it's $20. This will be my third box to the hospice group, and I just don't hand out $50 checks, but I'm doing more in that via postage.