I'm also new to sewing, but I've had great luck with
Joann Fabrics (I love to look through the remnants section-- a couple small bins full of fabric 50% off, and if it's on sale, you get 50% off the sale price). I also use their coupons, often have a 20-25% off coupon (even off sale stuff), 40-60% off, 75% off sometimes (specific items like buttons or thread). I think I've only paid full price for an item once. My mom works there, so she also gives me a heads up on in-store coupons and will grab a remant fabric for me if she thinks I'll like it. It's a blessing and a curse to have an insider!
Connect the blocks (online)
Keepsake quilting (online)
A notion to quilt (online)--shipping was pretty high, but they had a fabric I was looking for that I was having a really hard time finding in anything larger than a fat quarter. Even with the shipping, they beat the prices of the few other places I could find the fabric
Fabric Depot (when I'm in the big city)--they are expensive, but have some good sales sometimes
Thrift stores/second hand stores-- I've scored some good deals, even with a very limited selection