Joann's ALWAYS with coupons - and be sure to look at their red tag area. You never know what you'll find there and when they have it marked down 50% - WOW! I've actually gotten batiks for $2.50/yd.
Walmart. The last time I went there I was just looking for black fabric for a project. My darling husband was wandering around looking at prices on bolts and came up with quite a few that were only $1/yd. They were nice tiny floral prints and not at all thin. I took it all.
Estate sales and yard sales. I can't tell you how much fabric I've gotten at estate sales. At one I filled my car until I could barely fit in for $120 - and that included long bolts too. That was at a fabric designer's home. At a quilter's home (who was a fabric hoarder galore) the prices were a little higher, but I still got a ton of fabric and notions. Don't forget these sales as a source for thread. I now have 5 16qt bins plus two working boxes full of thread in every color under the sun. I've had to toss very few spools over the years because they were brittle. The going price around here for thread is between $1-$5 for a grocery bag full.