Once you exit customs and immigration at EZE airport, there is a Bank around the corner on your right where a lot of people exchange money. It has really good rates, so don't use the first Cambio Exchange inside the airport (it has far worse rates).
Once in Buenos Aires you'll get good rates simply by withdrawing money from the ATM machines here. I looked back in Canada and in Houston airport on my way down here and every exchange rate was far worse than the ones locally here, so I would wait until you arrive before worrying too much about obtaining Argentine Pesos.