I am in San Isidro and last year I made an extensive research on this topic, and the only place I found was in Tigre (Sixt) which was very out of reach by public transport. They offer a delivery service but they were unavailable to perform at the time because they were short on staff, so we had to take a taxi to go there. Also, we were renting on a Saturday which made things harder.
Most of the car rentals I found on the internet were either not responding or no longer renting cars, or simply with no car available when we called. I think the best bet would be to find one in Capital near public transport (e.g around Retiro) and my advice is not to be over confident on short notice rentals.