It´s known it as "col rizada" here and you can find it sometimes at those farmer's markets (mercados barriales) and in high rent districts. It's not common but it's around in some places. Both the green kind and the kind that's tinged with purple that is usually used in ornamental gardening in the US.
You can find it at the Sant Telmo greenmarket. They call it “Kale” but pronounce it like the city Cali in Colombia.
Very cheap. $30 pesos for a huge bundle. I also found it once in a Chinatown market.
The San Telmo greenmarket is on Balcarse St. from México to Chile street. Only on Saturday. I
Kale has become pretty common in the last 2 or 3 years, and I can usually find it at the green markets, and at several of the better fruiterias in my part of Palermo. And I see it on restaurant menus more and more, as well.
I buy it through "Mas cerca es mas justo" which is a non-profit that distributes "fair-trade" goods from local farmers and producers. https://www.mascercaesmasjusto.org.ar/como-comprar
You have to order through a Google Form and pick it up at a designated date & place.