My sister and I went to an all-town garage sale Saturday. I think we made more than 40 stops - we started at 8 AM and stopped around 4:30 PM
I found fabric (any at all) at only three places.
So - I would not recommend PLANNING on acquiring a stash at garage sales. Finding piles of fabric for cheap is in the same category as getting a high-paying lottery ticket.
Overall, I did make out like a bandit on Saturday, but my fabric "take" was comparatively small.
If people do have quilting type fabrics, it is usually gone quickly.
I didn't notice much craft stuff (knitting, embroidery, scrapbooking), either.