I'm not quite sure that I understand your question. If you are transient (such as RVers) you will find that many of the companies you sign up with have shops to be done in every state because they have nationwide clients. Examples of these are Bare, Market Force and Corporate Research. Other companies are likely to have clients with locations that are regional or the companies may be shopping just one region for a client. If you are an RVer and your next stop is going to be in Des Moines or Phoenix, I would suggest that you check Jobslinger.com (the free version) to find what companies shop the next location you will be visiting.
But then you state you live in Washington state, so I assume you are asking specifically who shops Washington state. Again, let me suggest the same companies mentioned above and Jobslinger as your starting point, but also look to signing up with more companies if/when you find that shopping works for you. There is a list of 200+ companies in the first post of the first thread of the Mystery Shopping Company Discussion section of the forum.