I noticed that some of my favorite merchants and informative folks aren't linked to, so I figure I should share.
Lacis- If your craft can be done with something that looks like a needle, they probably sell what you need and books on how to do it. This includes Lacemaking, Habedashery, Kumehimo, Knitting and Spinning.
Atlantian A&S Site- If you want to know how to do something from before 1650, you can probably find information on it here. Leatherworking? Stained glass? Obscure Russian foods from 1200? They've probably got it.
Vio Gemini designs- Corsets! Actually, there are lots of corset places on etsy, but i particularly like the fabrics this place uses.