I worked for an interior designer and we used Linensource.com for a lot of our bedding needs. They're cheap overall, and they have good clearance sales too. Quite a range of comforters and bedding, in all different thread counts etc. Give it a shot.
A twin comforter should fit just fine. Like you said, there are not many XL comforters out there. My kids used regular twin comforters and it looked fine. Good luck at college!
You're right when you say that polyester mixes pill over time. However, this truly only applies to sheets. As far as comforters are concerned, they won't pill as quickly or as much, especially if you take care to remove the comforter from the bed whe...