Masters logo apparel is only available at the gift shops at the tournament - it isn't sold anywhere else officially. But browse eBay afterwards and you'll find a lot of stuff being resold by patrons.
They do have an amazing selection of clothes in the shops. There are at least 40-50 different men's shirts and a couple dozen women's shirts, plus dozens of pullovers, sweaters, vests, etc. The shirts range from about $60-150, and the sweaters I saw were about $80-150. I was there for Monday and Tuesday and got out of there with a restrained $400 in goodies, but one could easily go crazy.