Craigslist can be a good source. Also check newspapers for garage sales that mention quilters or fabric. Google freecycle to find a freecycle group in your area. You can usually post an "Ask" once a week on freecycle. (Freecycle is for recycling goods for free. Mostly it is for people who have something they want to give away.)
Also, try joining a quilt guild. You can do a "show and tell" of one of your quilts and mention that you are collecting fabrics for more quilts, or you may be able to place a notice in the guild newsletter.