No barber shops that I know of, but back in 2007 I had my dreads taken care of by Andres (http://www.natural-rasta.com.ar/index_v2.php). Great kid, skilled, and well priced (back then, don't know what he charges now).
Anyways, beware of an old lady called "Coty" that pops up on top of most searches online. She's a con.
Check out the Dominican neighborhood in San Cristóbal/Constitutión. I know of a place off of Av. Independencia somewhere between Virrey Ceballos and San José. My "black" dominican friend always got his hair cut there, as well as braided. Expect free beer and reggeaton music when you're in there
they always braid hair at Parque Rivadavia. I have seen guys from Jamaica doing great work. I just saw them today passing by. You just take the Line A of underground (subte) and get off at "Acoyte" exit and on that same street you will find them if needed