I don’t live in Portland but have gone record shopping there many times and would think your best bets are:
Music Millenium
Everyday Music
2nd Avenue Records
MM and EM are both huge and always get tons of RSD stuff. 2nd Ave is a little smaller but right downtown so depending on where you’re staying may be a little more convenient.
Jackpot is is pretty good, too, come to think of it. I’d just map out whichever of these four is closest/most convenient for you and you really can’t go wrong.