Here are some of my favorite resources for the things I make. While I prefer to buy locally (as in I want it right this second), there are just some things that I can't get around here. I have scoured the web and found a few places out there that I want to share with you!
Sailrite is a great place to buy fabrics and sewing tools. They specialize in marine applications (such as sailmaking) but there are a lot of great how-to videos for projects around the home. Their videos are well made and easy to follow, I highly recommend!
If you make bags, Strapworks is a great resource for all things...straps! They carry many varieties of nylon webbing (think backpacks), as well as good hardware (metal and plastic) at a reasonable price.
Making anything with grommets means needing grommets. Grommet Mart has a great selection beyond the standard Dritz grommets you can find at a local Joann Fabrics.
If you're local to the Chicago area, check out Vogue Fabrics in Evanston. They have a huge selection of fabrics for apparel. They do have a website and you can order online, but if you're local, check it out, it's worth the trip!