What type of milkweed can be grown in the Pacific Northwest to benefit Monarch butterflies?
Western Oregon's native milkweed (Asclepias speciosa) appears to be the most beneficial plant for Monarch butterflies in the Pacific Northwest. Here is a link from Oregon State University that will provide you will more detailed information:
In addition, there is an organization called the Oregon Milkweed Project which seeks to encourage the planting of native milkweeds. This might be a good resource for you as well. I noticed that the organization's Facebook page lists an upcoming milkweed class at Portland Nursery on SE Stark St. (March 14th). You might be interested in that as well.
I am confident there are several local nurseries in our area that carry native milkweed. Your best bet is to call some nurseries in your area and ask if they specifically carry the variety Asclepias speciosa (common name: showy milkweed).