Not exactly in the centre of town but Carrefour San Isidro (outside San Isidro, on the intersection of the Panamerican and Camino de Buen Ayre) has it. Look for the aisle with the powdered soft drinks and the aisle with spices and it's the one between those two. The shrimp paste (I think it's Lee Kum Kee) is near the bottom of the display unit (also has soya sauce, oyster sauce, chilli sauce etc), which is opposite a unit which has anchovies and that kind of stuff if you need to ask for directions from a hapless employee.
