Good question, because there is a wide spread. For example, dolarblue.net is showing 14.2 compra / 14.9 venta right now, and lanacion.net.ar is showing 15.4 / 15.55.
This open Facebook group is very good for when you want to buy/sell dollars or euros with no middle man involved. Just individual people who want to trade foreign currency for pesos (I've used it a few times myself):
https://www.facebook.com/dolarblue