You should be warned that certain métro stops seem to NEVER stock the card; Solferino, near the Musée D'Orsay is one that hasn't had them a lot, with a hand-lettered sign saying "go to another station". I just think they don't want to spend the time necessary to sell them, as it's usually manned by only one clerk. And the Musée D'Orsay RER station doesn't sell them, either.
I know that last week when I stayed a block away and used Solferino station every day, the situation never changed; be smart and get one wherever you're getting ON the métro.