If you're in the Portland area, here are the best places to get...

Orchid Bark: Rexius. Their small size bark is especially nice, but the medium size is ok too. $12 for a 2cf bag. Rexius also sells Perlite! So if you're already there, make sure to grab a 4cf bag of perlite for $15 (the XLP). Ever since Concentrates moved way the heck out to Milwaukie, it's been a major pain to get to, unless you're also driving to Bob's Red Mill or Dave's Killer Bread, which are right next door and worth a visit.
Perlite: Concentrates. Their extra large perlite (XLP) is great. $15 for a 4cf bag. They also have big bags of Peat.
Pumice: Oregon Decorative Rock. Comes in 1/2cf bags for $4 each.

If you're growing lots of plants, then you need all of these things desperately! For the best drainage, plan on using a sieve to get out all the fines before use (I use plastic replacement screen door fabric that's specifically designed to resist pet scratches, available at a hardware store). Your average epiphytic orchids will do well in 1 part perlite and 1 part orchid bark. My standard well drained mix is just a 1:1:1 mix of all three. Works great for terrestrial orchids, succulents, native and exotic bulbs and everything South African. Just add half part or one part peat moss and it's now the perfect well drained potting mix for everything else in the world... epiphitic

BTW, I've recently needed a goodly amount of oyster shells to make my mixes have a higher pH. I've found small amounts here and there (Portland Nursery on Division even had some, as does Concentrates), but to get a big bag that will last a while I went to my "local" farm supply co-op (they have several locations on the fringes of Portland: Wilco co-op. 50lb bag for $11. They also had the cutest little chuckar chicks for sale... as big as your thumb!