If you live in NYC, I'm sure they are available in Chinatown since the Momofuku cookbook references that they purchased them from a wholesale shop in Chinatown, and the shop and others probably sells to stores, but I've only purchased frozen dumplings, not bao wrappers so I can't tell you offhand.

I don't know what's available frozen, but you should definitely try making the dough yourself! It's fun and easy!! Here's one that looks pretty easy: http://chinesefood.about.com/od/dimsumbuns/r/steamedbun.htm

Good luck!