Here's a few sources I have either used or come across for parts, including odd things such as 5 and 6-way 4-pole rotary switches, chicken head knobs, and push-pull pots, unusual mini switches, center detent blend pots, and finishing supplies.
Of course,
StewMac.
Tons of components and switches at
Gregg Rogers Guitars.
More components at
GuitarElectronics.com.
Guitar Heaven has some decent prices at times and is quite easy to work with. I have bought Duncan pickups from there and when I buy two there is no shipping cost ($100 and over free shipping).
One of the forums regulars also came up with
Luthier's Mercantile International.