There have been many good sources listed on this thread.
I would also encourage you to join a guild or charity sewing group. Our guild periodically has an informal in-house auction open only to members. Here members donate fabric, quilt tops, blocks, patterns, and other quilting supplies. The proceeds go to the guild, and most of the fabric ends up being very inexpensive. We also do "fabric exchanges".
Charity sewing groups frequently get fabric donations. If you sew with them, you can use their fabric for charity projects and sometimes trade something from your stash for something you can use on personal projects. Sometimes they just want to give pieces away to anyone who can use them.
Quilting with others opens many doors. If people know what kind of fabric you need for a project, they will frequently give you something from their stashes. You will find mentors who want to encourage you in your quilting and will help with supplies.
I think the rule of supply and demand applies here. If you want to buy something inexpensively, go where there is an abundance of that item owned by someone who wants less of it (or doesn't have space to store it).
Dayle