Here is a great reference page with a list of butterfly species for Whatcom County Washington. By clicking on the items you will link to the page for that species. On that page you will see a detail "Caterpillar Host".
Finding some of those plants may be challenging, and then some will be easy. The more challenging (imo), the more endangered that species may be. I will say that being on davesgarden, I have always been able to find what I needed. http://www.butterfliesandmoths.org/map?dc=3921&_dcc=1&si=48
If you have any questions on the plants or a particular species, please don't hesitate to discuss it with me here. I will support you in any way possible.