I have a skinny riding pal - he's 6'4" and 145. He gets custom fit jerseys from Boure, out of Colorado. They charge a 25% premium to do custom, and it's high quality stuff. Their classic jerseys are on sale now for $45 - they're normally $60. Not sure you could get the sale price on a custom order.
http://www.boure.com/customfit.html