Tire dealers in BC, Washington state or Oregon.

Have you thought about buying on EBAY, Tire Rack or Tirebuyer and having them held for Pick up on U.S. side? You can get them mounted and balanced at any good tire dealer. Tire Buyer is my online favorite.
As for tire stores I have no knowledge of the area you will be in But my go to store is usually BIG "O" tire and so far I have had good luck with them on price ad service wherever I am. This also gives me somewhere to go in a different city should I have a problem