Don't a few of the dive shops rent dry suits? I don't live in Seattle (yet, probably will if I can sell my house) but when I was checking out dive shops in the area I thought I saw what looked like rental dry suits at one of the stores.
Oh, one other thing, you might want to switch over from Aluminum to Steel tanks if you can. The first time I went dry suit diving in the Seattle area I need I think 36 lbs to sink me in my dry suit with an AL tank. Granted with some practice and likely some better underwear I can probably get it down, but still the weight was shocking. Its quite a difference from fresh water Great Lakes diving where real cold weather is 28 lbs, and as low as 16 lbs in the summer. Its probably why a backplate is so popular there over a conventional BC.