You'll want to call them first to make sure, but there is a great outdoors store on Yrigoyen, right near 9 de Julio called Wildlife. I would think they would have it. That said, I have tried a two-person hammock - have you? I found it remarkale uncomfortable!!! But good luck just the same. Also, along Parana mear Rivadavia, there are four-fice camping stores, you may want to try there if Wildlife (a great store, very helpful staff, some of whom speak English) doesn't work out.
thanks so much guys! i'll try out those stores wreReynolds and search that Tigre market! And no, I don't think I have tried a 2 person hammock, but it's a gift for newlyweds. And I'll def go on Amazon or Ebay if I don't find one, it's just more authentic if it comes straight from here -all the ones online are Mexican.