Wellll.... there are a LOT of quilt and fabric stores in and around Portland, and they all have different "flavors" - i.e., one store specializes in modern prints, another carries nothing but repros. There's even one that focuses on flannels. Most of them do a fairly good job of carrying a little of everything. There's a store in Vancouver that carries nothing but Moda.
Most of the quilt stores are outside of the downtown core area and would require driving. I agree with Dunster, Fabric Depot would be a great place to go if you can only hit one store - it's massive. It's on the SE side of Portland, 122nd and Stark.
If there's anything specific you're looking for, or you tend to like a certain "flavor", let me know.
Fabric Depot is listed on the "Row by Row" website as a participating vendor. As long as you're this close, you may want to hop across the river to Vancouver and get a Washington plate, also.
Oregon shops:
http://www.rowbyrowexperience.com/oregon.htm
Washington shops:
http://www.rowbyrowexperience.com/washington.htm