Hi Mary,
In addition to Community Cycling Center, there is Citybikes.
http://www.citybikes.coop/index.htm
My housemate got a great bike there, pretty much custom built from old bikes. You can mix and match. If you get the right frame, you can start with upright handlebars and then if you change, replace them with drops or whatever. Frankenbike... Even if you get them to do all the mechanical work, the bikes are still a great deal. They also sell some good quality, moderately priced new bikes from Jamis, KHS, and Biria. What they don't stock, they will order.
If I hadn't been so set to buy my Breezer, I would have gone to Citybikes.
BTW, if you're a member of any Portland food co-op, they'll take 5% off your bill.
Couple of other places that sell used bikes:
http://www.therecyclery.com/
http://www.sellwoodcycle.com/
Congratulations in getting off Tri-Met!