These online stores are ones I use myself, and have been very happy with their service.

Fabric/Accessory Stores

Fabric.com - fantastic for your basic fabrics.
Silk Connection - good range of undyed silks.
Thai Silks - extensive range of silks.
Denver Fabrics - good for all general fabrics.
Birka Traders - replica brooches, pins, clasps, buckles, etc etc.

And for everything else, there is eBay!!