There is one in Galeria Embassy at the beginning of Florida near plaza San Martin. If you go in, the guy who exchanges money is not there, he is also cautious. There should be someone near the entrance of the galeria saying "cambio cambio". If you follow this person, he should lead you to a shop in the galeria and the guy will show up. I have always had a good price there with 100 dollar or euro notes.
I have also exchanged with other random people elsewhere in Florida and never had a problem. I mitigate the risk of something going wrong the following way: I never take too much money with me, only exchange 500 USD or euro at a time and I always carry a bag in which I can discretely put the money. You get too many peso notes for 500 USD nowadays to be able to put them discretely in the pockets of your jacket, but a hand bag might do.