I look under "garage sales" in Craig's List and go on (name of your city)garagesales.com. If there is anything on there that indicates they have fabric, craft items or anything else that I want, I go. I will often go to those labeled "estate sales" also.
I know there are garage sales that do not advertise and there may be fabric at some of them but I don't go to as many sales as I used to. My best find, I think, as far as quilting goes is my Accuquilt Go for $75.
I have also found quilting quality fabric at Goodwill a a few times. I haven't found any at the Salvation Army Store.
As in any catagory, at a resale shop, you take what is there when you find it. Otherwise, keep looking.