Hellobee Boards

You might have better luck in summer/fall? They usually come out as kids are going to college.

Wayfair!

Edit: I just saw your comment and had to double check. You are looking for a twin XL, right? Wayfair has a few to choose from. https://www.wayfair.com/bed-bath/sb1/twin-xl-duvets-duvet-sets-c215335-a135976~449505.html

Pottery barn teen! If you go to the “dorm” tab, all the descriptions say that they fit twin XL even though it says “twin”’for the size

Try http://www.thecompanystore.com ? I just bought lo1 some twin fitted sheets from there (since she doesn't use the top sheet or duvet, weirdo lol) and the sheets were on sale