We bought the Museum Pass (both times we've been there) at our first stop into a Metro Station. Just go up to the ticket booth and ask for it. THey are very used to selling them. We'd also buy the pack of metro tickets (can't remember what it's called, but I can try to look it up) as we used the Metro A LOT. You can also buy them at almost all the places that take it.
What I liked about the Museum Pass is I tend to balk at paying fees to get into places over and over and over. We really liked paying once and then putting our wallets away for the rest of the trip. You never had to stand at the entrance to an attraction or museum and ponder over whether it would be worth the entrance fee or not. Like going up to the top of the Arc de Triomphe. I think it was something ridiculous like 10E per adult! CRAZY! No way would I pay that. But with the Museum Pass you just go on up and enjoy the view and not have your husband grousing "this is such a rip-off!".
Because of the Museum Pass, we went into many many many places that we never would have actually paid for because they were "free". Purely psychological, but we made some great discoveries of things we never would have experienced without it........the Cluny Museum, the Pompidou, the Picasso Museum, St. Chapelle, the Conciergerie.... When we took my mother-in-law back 2 years later, the kids enjoyed taking her to all of these places and we popped in and out and enjoyed them again for "free".
Plus, you can go into a museum to "try it out" and see what you think and not feel like you have to spend hours and hours looking at every darn thing to "get your money's worth". This was huge with our kids along, because we could wander through and look at what caught our fancy and bypass those that were more ho-hum. We felt like quite the explorers and our kids LOVED all the museums because it wasn't torturous. We popped into the Rodin Museum for a quick look-over and then out to the gardens to check them out. Never never never would I have paid money at the door for it, but it was sure great to see. We thought we didn't like Modern Art or Picasso, but after trying out both the Pompidou and the Picasso Museum on a rainy day we became quite the fans! (and even if you still don't like modern art after going to these, you'll get quite the giggle out of the other folks that are taking some VERY crazy off the wall art VERY seriously!!!!) This kind of experience happened over and over.