Found while browsing through the goods at Despaña in Soho: a Horchata Chufi. The horchata I'm used to drinking is made from almonds and most often found at Mexican places. This Spanish version of horchata (or, as the lady behind the counter told me, "the real horchata"), is made from chufa, more commonly known as tiger nut. It tastes nothing like horchata made from almonds. Horchata Chufi is significantly richer, and positively velvety if you go for the Horchata Chufi Maestro, which is made with a higher percentage of tiger nuts. Take note: the tiger nut isn't actually a nut, but a small, starchy tuber, with a flavor like that of a slightly sweeter chestnut. If the taste is oddly familiar, it's because tiger nuts are also used in traditional Chinese soup desserts.
Purchase the horchata by the liter for $5 for the original and $7.95 for the maestro version. And if a liter is too much, they also sell it by the cup for $3.