Here is a list of sources for Fabrics, Hardware, Trim, Grading, and Sewing Notions. This list will update periodically.
Retail = 40 yards or less. “Piece Goods”
Wholesale = Up to and exceeding 1000 yards. Also Sample Yardage (usually with a Cutting fee if less than 100 yds. estimated)
You should buy wholesale to ensure whole-salability of the material you use for your design.
Most of these sources both retail and wholesale materials.
Hardware & Trim
Amazon (lots of hardware can be found right off Amazon)
Etsy (same goes for Etsy, LOTS of sources for many things)
Ace Sewing (LA)
Sil Thread (NYC)