There is a real difference between fabric and if I'm going to put that much work into a quilt I want it to last and feel good when I'm done. The quality of fabric at the major chain stores is not anything I would put in a quilt. I always check the sale rack at any quilt store I shop at. It is not always evident where it is - usually in the back. If I can't find it I ask. The fabric there is usually fabrics that haven't sold well or the last bolt in a family of fabrics etc. Often times it will work for me.
Another reason I like shopping at a quilt shop is that the persons helping me have a passion for fabric and quilting and are excited about my project too and have good advice. I don't find that service at a chain store.
I also belong to a quilt guild where they have an exchange table. Members bring in fabric, books, magazines that they no longer want and others can take.