I'm the the Portland metro area. There are tons of poly people in the area. Corvallis has a poly group that meets monthly, I know, although I have heard mixed reviews. Salem has a kinky munch that has a large helping of poly folks, as well, but it is definitely a kink group, not a poly one. Then Portland has quite a few different offerings, but that becomes quite a drive just for an event.
OkCupid is an awesome resource. Lots of poly people in the area use the site.