The ones at the taco truck on 97th St. near Broadway are now $2 a pop and are only available on weekends. They're the really fat kind, though, and usually very good, with lots of filling.
La Noche Mexicana on the UWS has decent ones--skinnier and less expertly wrapped--for the same price. You buy them in threes.
For both places, I'd call ahead and warn them that you'll want a lot. They tend to run out (and they might even give you a discount for bulk buying).