I highly recommend Romashka Bridal. It's a little bit of a drive north to Everett, but so worth it. Their selection is MASSIVE and they do customizations and alterations in house. The location isn't elegant, and their building isn't fancy but their dresses are GORGEOUS and they offer a really impressive range of sizes to try on.
If you're looking for a great deal, Eltsa Bridal Alterations has a TON of dresses from a closed bridal salon. They are all samples not worn for a wedding.