The best rates i've ever seen were in tourism agencies (with no currency signs) around Florida street, sometimes they mask for old coins shop. But you need to ask in Spanish (ask every man who is shouting «Cambio») and do some research about today's rate brackets (sell/buy). USD 100 & 50 bills are more expensive than smaller ones.

And even changing small amount right on the street was not a problem (no fake bills, etc.), except lower rate (agreed beforehand) after 6 pm or on weekends. ATM rate (not counting extortionate local fees), i.e. pure MasterCard USD/ARS 0% fee rate could be the same, but sometimes up to 5-6% worse (than for cash).

Another good reference is exchange rates in for Encarnación and CDE in Paraguay, not legally bound to Argentine laws.

Verified last time June 2018.