I do this! Every Sunday our Goodwill has one color tag on sale for 99 cents and I go every week and scour the store. I have gotten some really cute fabric this way, and I just started a button jar. I wash it right away and then cut it up sometime during the week and put the buttons in the jar.
I also look through the special occasion clothes because I like to embellish and it's a good way to get cheap, nice beads or sequins.