I think the McCall's patterns website offers a plus-size tank as one of the free downloads its members can get. It's a free membership.
HotPatterns offers free downloads from time to time, and I have seen a tee-shirt in the past.
Try looking for a free online pattern called Sorbetto. I've seen lots of nice tops made from it. It's sleeveless, but someone made a pattern for sleeves for it. Not sure how big it goes, but it's so simple in style it ought to be easy to enlarge it.