Some popular/good venues in Portland are not limited to:
The state theatre, port city music hall, the asylum, and a hundred more.
Lots of bars and clubs hire bands. The Big Easy is good, Bull Feeney's, Rira, and Brian boru's all book bands regularly. There are a bunch that I'm leaving out.
Check out the Portland Phoenix magazine for a full list. They publish most of the city's events each week.