I know that it's possible for foreigners to obtain the RFID transit card SUBE but I don't know where to get it. It seems that it's not sold in all Subte stations.
Where can I get the SUBE card?
You'll want to go to a locutório (like a small shop). It'll be called that - "Locutório" with a purple sign saying "SUBE" in the window.
Where are these shops? There's a handy website that maps all of them in Buenos Ares.
Then according to a blog on the subject:
You can get one in almost any corner(convenience) store that. They will either have a sign saying that they sell "tarjetas" or you can ask them.
FYI don't load too much money on your bus card at once as certain cards only work in certain cities.