Unless you are a non-resident of Canada, the official CCRA line is you cannot rent a non-Canadian registered car outside Canada and drive it into Canada.

There are supposedly exemptions for short one day rentals to get home but not to drive around Canada before returning south. Other exemptions are people returning to live in Canada, purchasing and moving stuff home (e.g., buying furniture), emergencies (such as your car being stolen, wrecked, etc.), and being a non-resident of Canada.

As for Canadian cars - yes, they are far and few between in SEA (found that out a few times this past year). Make reservations at Hertz, Budget, Avis and National and hope for the best (National used to be cheap but you're looking at $100+ basic rates these days) and offer to swap for a U.S. car when you get to YVR. FWIW, National in Canada told me an average of 50 of their Canadian cars are driven OW to the U.S. yearly.