Maybe I am wrong of course but it seems less likely to me that I will get a bad bill at a bank. Also bank branches are so easy to find. Exchange places, in my experiences, not so much. Anyway ... yesterday I tried many bank branch offices as I walked down a stretch of Las Heras. Not a one would exchange cash for a non-client. So I wandered my way to an unofficial exchange place I have used several times in the past near the intersection of Ortiz y Cerviño (on Ortiz next to the Freddo).
Thanks for the several responses.
Bob