I've been doing this long enough (and have been a compulsive sewer long enough) that I can hold off on buying fabric until I save the money for good fabric (there's an online source for 100% linen that's cheaper than anything I've found in store in my area). Since I wont use man-made fabrics AT ALL (not a period issue, it's a safety thing. Natural fibers will smolder before they burn. So if you're standing next to the fire when a knot spits, you get a small burn mark on your skirt, you don't go up in flames- which I've seen happen. Scared me out of my wits), I tend to wait for the money to make good pieces, that'll last. But, like I said, I've got the garb built up to get away with that :}.
But, your local thrift shop can be an excellent source for swathes of cotton (they call them queen and king-sized sheets, what do they know!), and sometimes some really nice upholstery fabric (old curtains
). And surplus stores usually have wool blankets for cheeep. Lovely cloak material
Yes, I like natural material as well, and I HAVE found linen (!?) on the $1.00 table (and bought it all)
You get really, really funny looks out in the parking lot doing burn tests
(after I figured out how to work the lighter I had just bought, silly child safety thingies, just about defeted a 47 year old)
you just have to learn to feel the difference (I love to go to a fabric store and "pet" material, just need better ones around here, it seems all ours have anymore is fleece)