I don't know of any barns that currently are taking working students. There is one barn in Woodinville that you should RUN not walk away from. If I knew how to send you both private messages, I would. I don't want to bash them widely on a public forum but you should stay far away. If you send me messages maybe I can figure out how to respond to them! :)
Good barns in the Redmond area for dressage/eventing would be:
Saddle Rock Stables (eventing). I haven't ridden there but I liked the instructor who is young but seems quiet and thoughtful. There are horses for lease last time I checked.
Dressage Dancers - this is a very low key barn in Woodinville. It's mostly beginner oriented, but the woman who runs it is very nice and there might be a possibility for a working student position if you're a more advanced rider.
Hilltop Stables- eventing barn. Not sure about their lesson program. They do take EXCELLENT care of their horses though.
Riders Edge Farm- the people were really nice but board is pretty expensive and I think lessons were also kind of pricey.
There are some super fancy dressage barns that I haven't visited since I know they're out of my price range, but you might look into the working student thing. I know that San Sebastian Farms (Lisa Copeland) is looking for a groom but I don't know anything about her or her farm. It might be worth checking out. The ad is on craigslist.
Thank you for the list! I started googling, and the first one looks promising, I emailed her. Also, thanks for letting me know about that barn. I tried to reply to your message, but apparently I haven't posted enough to use the PM feature (I'm more of a lurker). I've actually seen ads on Craigslist from that barn, and I thought it seemed sketchy.