Hi All
There are a few places that are likely to allow free cash withdrawals for foreign cards as per this forum.
Max has found that a lot of ATMs are not available outside of office hours when the bank is closed. There are some non-free but constantly accessible ATMs in the Bellas Artes metro station.
Here are the nearest probably-free ones to the hostel, if you are happy to have a short walk:
Banco Internacional
Banco Security