http://www.webcyclery.com (Bend, Oregon, woo!) for single speed and trials goodies online. http://www.biketoolsetc.com is out of Ashland, and they have those miscellaneous bits you can't find in the usual places.

In Portland, Veloce (http://www.velocebicycles.com) on Hawthorne has been hooking me up with my track stuff. He also will build a singlespeed, if you ask him.
Bike N' Hike on 400 S.E. Grand stocks some track and BMX stuff.
Gresham Bike off the MAX line used to stock some trials stuff, but now I think they mostly do BMX and hybrids.