i look for cotton wherever i go. if a poly mix is really cheap/appealing, i get that and add to the stash of that fabric. sometimes it's nice to have something indestructible for things that will get harsh use. and wool! i go for wool. it can be felted and used for throws and appliques. it can be used as is for warm car quilts. mittens, too! trace and blanket stitch after you sew on an applique.
yard sales are great--estate sales even better (if someone there was in to sewing). craig's list, too, as well as free cycle. and the day after a garage sale closes (or the night of it) they often just want to toss whatever is left. any fabric is game--shirts, etc. creativity is where it all started, in the sewing and the fabric acquisition.
an old friend of ours made rag rugs on her loom in the basement where she lived with her son wendell, and daughter in law. she was in her late 80s when she passed away. the last time i saw her she confided in her cute swedish whisper, "you know--it takes five pairs of wendell's pants to make a decent rug..." and wneh i mentioned it to him, he grinned and nodded. "she shops where she can find the fabric. my closet is on the list."
fair game, ladies and gents. it's ALL fair game.