Florida street, between Cordoba and Corrientes is the easiest. Look for guys shining shoes or standing in a doorway, make eye contact, if they say "cambio" quietly stop and ask them their price.
I know I'll get a lot of people telling us how dangerous and undependable it is and all I have to say is
(I've been doing it for the 6+ years I've lived here - I rarely went to a real change place even when the blue rate and official rate were very close).
Having said that, there are other, more established places, but I don't bother to try to keep up with them because they tend to come and go and the guys on Florida are dependable and they take you to places away from the public, but not in an unsafe manner or place, much like any other place like a fake jewelry shop that does currency conversions.
If you see people in "AFIP" jackets walking around when you go, give up and come back another time.