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'll fill out a small application with your name, address, and passport/DNI number and pay the $20 ARS. However, your card will not come loaded with funds and the locutório may not charge SUBE cards. So you will have to find a kiosko that does. There are many along Santa Fe that sport the SUBE logo in their windows. These places may or may not be able to charge your card at that time, so again, you'll just have to ask. When you find a charging station, you can have up to $50 ARS put on the card at a time. The clerk will charge your card on a space age sensor machine and hand you a receipt with the proof of funds.
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.