buy mobilis pass at St. Sulpice metro station?

Mobilis tickets can be purchased at any machine. Be aware that the machines only take coins or chip-and-pin credit cards (no paper notes). Also, I remember at least some machines only had Mobilis tickets on their French language pages, not their English ones. I also bought Mobilis tickets from ticket windows, but others are posting that not all stations now have open windows (I have not been in Paris for several years, and when I went this wasn't an issue). You must specify the number of zones when you buy the ticket, but you date it yourself (have a pen handy). Be aware that the airports are specifically excluded from Mobilis tickets, even if you have one that includes the zones the airports are in. I love the Mobilis ticket; it's great for someone zipping around a lot on a particular day, and particularly for going to Montmartre, as the Montmartre funicular otherwise requires a separate ticket.

Merci. Lots of wonderful information!

I'm not sure whether St. Sulpice is a manned metro station, but you can purchase the Mobilis tickets from the machines (coins or chip & pin only). They can be tricky to find - especially the 1/2 price weekend ones for youth - and you might need to check under the "other tickets" option.

I bought tickets at ticket machine with my regular American credit card. Guess I got lucky.