- Online Metals
The best place I've found to go for arbitrary length raw materials of arbitrary types of raw materials, second only to McMaster-Carr (but way less expensive).
- Aircraft Spruce
For all your performance-grade materials delivered cheap, check out Aircraft Spruce. Their primary market is people making hobby aircraft, and their selection shows.
- Exotic Wood
All kinds of hardwoods available online. I've never ordered from these guys, I just know they exist.