What we need to clarify is that under no circumstances should visitors use foreign credit cards.
As far as I can see if you don't have a DNI which could allow you to open MercadoPago or a bank account, the best exchange rate (other than an "arbolito" on the street) is sending funds via WesternUnion which as of today was...1.00 USD = 170.6800 Argentine Peso (ARS) while Xoom which also has cash pickup is at 1 USD = 162.3194 ARS. Another side note, many stores don't have more than 200 USD equivalent even at mid-day so might help to call a store in advance to see if they have the funds. (Issues in Palermo/Belgrano) Planning on writing an article about it as starting in November? Neighboring countries can visit...