Our families favorite spot for cakes is easily Hoffman's in Kirkland. Every one I've tried (white, chocolate, etc.) has been delicious - moist, dense and with amazing frosting. It will certainly cost you more than Costco, but it's well worth it for a special occasion.
The store itself serves sandwiches for lunch, and has a wide array of pastries to choose from. I haven't sampled the pastries, but the lunch I had there with my mother-in-law and my 18 month old daughter was quite tasty.