Go to El Paso Taqueria on 103rd and lex. They also have another branch on 97th street. It's Mexcian, but fantastic and cheap too. I have brought many a skeptcal friend there, and they all rave about the horchata.
Any down home Salvadoran or Mexican place has orchata. I have had MANY a pupusa and taco washed down with a tall, sweet glass of it. Imagine how many places on Roosevelt Ave. in Queens serve orchata,I can't count that high.