Milk Bar has an outpost at the Union Square Holiday Market where they're selling stollen. It's my only experience with "fruitcake" so I can't say if it is traditional or not, but it is good. There's some talk on the board that it was better last year. I guess that's irrelevant now, though.
Momofuku Milk Bar
251 E 13th St, New York, NY 10003
Union Square Holiday Market
E 14th St Union Square W, New York, NY 10003