St. Thomas has Fabrics in Motion. I've stopped there the 15 or so times I've been at St. Thomas. It is right off the main street by the water. I believe the Green House Restaurant is on the corner. On that same block is Rosies. That shop is actually right behind Fabrics in Motion. That also has some "craft" items. Seems like I got yarn there one time when I needed something to do on the ship. Pizza Hut is on the "water corner" for Rosies.
San Juan use to have 4 or 5 fabric stores near where the ships dock but last time my husband and I were there they had closed or moved. Didn't find any closeby when I went in Nov. with my kids.