I agree about estate sales. They are wonderful for low prices and can have not only nice quality current (or nearly) fabrics, but they are a good source for vintage fabrics also. Garage sales can be good too. When I go to them, I always ask if they have fabric or sewing machine attachments. It is amazing what comes out of the house when you ask.
I frequently check the newspaper, Craigslist, Thrifty Nickel, and the estate sale websites mentioned in an earlier post. Each community seems to be different in where estate sales and garage sales are most advertised.
The other wonderful source that I have is the guild and charity sewing groups. Not only does fabric come through them, but the friendships that I have made with fellow quilters often results in opportunities for trading and giving each other the fabrics needed for projects.