I used to buy almost all of my (the ones I didn't tie myself, that is) from Yager's flies (www.yagersflies.com
) they had excellent service, quality and were reasonably priced. I have only bought a couple dozen or so from them in the last year so I don't know if any of these qualities has changed. I do know that they have branched out into selling things other than flies but I have never purchased any of this stuff from them and cannot recommend them for anything but flies. They have a huge selection (500 plus patterns) so you might want to check them out.
One thing to note... their flies, as with many others (including Umpqua) are tied in Sri Lanka. There are some very high quality fly tiers here in the US if you prefer to support US tiers.