Because I work mostly on historical reproduction outfits, the sort of things I'm looking for need to be natural fibers and colors that would have been available in common use in the 16th century. That means wool and linen -- lots of it. And that stuff is expensive, especially when you're a stickler for 100% fiber, and try not to use blends. Wool or linen can easily run $20 a yard or more. Considering that it takes anywhere from four to seven yards of fabric to make a dress, that can be a huge expense!
I'll admit it's a rare day when I find seven yards of anything at Goodwill, but I have. And the price range I pay there is generally between three and ten dollars. I once made a terrific find of seventeen yards of handkerchief-weight linen for fifteen bucks.
Do you believe that? Wtf seventeen yards for fifteen... freakin... dollar bills. It's madness!
Shopping thrift is different than running to Joann's because you need a few yards of blue wool to make a tunic. It's more of a hunting-gathering sort of thing: you go to see if there's anything you can use, not just for a specific project.
In one visit, I might be looking particularly for linen to make a shirt. So I check out tablecloths first. A good sized tablecloth of 100% linen isn't as rare as you might think; folks donate them all the time because ironing them is a pain, or gramma died and left a shit-ton of them, or a catering business just closed down. The secret to finding the real thing and not a polyester blend is to look at the edge. If it's serged (bound with stitching), it's fake. If it has a turned hem, it's probably real linen, and if that hem is two inches and has mitered corners, you can be 99% sure it's real --If you can't tell simply by the feel and look of it. Once you've handled linen a while, you'll know the real stuff anywhere.
So I go through the tablecloths and maybe there's nothing good there. Onward to yardage. The 70's must be well and truly behind us, because the bane of polyester reeking of mothballs seems to have passed. For a while there was a dearth of the rotten stuff being culled from dead grandmothers closets and donated to thrift stores. One place I go to is nice enough to put labels on their yardage with the piece's measurements, but usually I have to unfold it from the hanger and guesstimate it. Maybe it's a lucky day and I find something that will work for me, but usually I don't. On the other hand, there is a piece of good wool, or three yards of an interesting brocade. Not what I'm after today, but something I could use on another project.
This is how one builds a dangerous stash.
Other things to consider: The bedding section, and curtains. Great places to find wool blankets (or large pieces of wool yardage that someone in back mistook for a blanket) or cotton velveteen or brocade draperies (but not the rubber-backed ones, or the polyester) and also skirts and dresses -- especially the plus size ones. There can be a nice amount of fabric in one of these.
National chain thrift stores usually have sale days, too, so watch for those and it's wacky what you can get for 99 cents if you're lucky.
Follow these simple steps, and soon you can be like me, with three dressers full of assorted wool, linen, brocades, leather, velvet and silks! ...but none of it exactly right for the project you're making now.