I used to live in the Seattle area. Whenever I went to Pike Street I would make sure to wander through Undercover Quilts. Nancy's Sewing basket used to be a fun shop, it is in the Queen Anne neighborhood. For a large store, I would suggest Pacific Fabrics. I just love the merchandising they do in there stores.
It has been three years since I lived in Seattle so you may want to check web resources before heading out to these places.