Paris

Did you mean Conciergerie? Yes,but the Pantheon is likely easier.

You can buy them at the sites included in the pass. And if the Pantheon is convenient for you, that one likely has the shortest line, if any

Reported the ad between this post and Jules' post which bumped this post up.

I'll add for the OP that you may also be able to buy the Museum Pass at a Tabac near your hotel. Certainly Pantheon and Concergerie will also be easy to buy it at as well.

I'd also recommend you look carefully to see if this is a good investment for you. I just got back from Paris on Saturday and "always" get a Museum Pass. At this time I'm recommending people just buy a timed entry to the Louvre instead of jumping thru their hoops to get a reservation with the Museum Pass. The Louvre may or may not be on your list.

Also be aware the Cluny and Orangerie Museums are both under drastic reno and have most of their collections closed. The Orangerie has only the Water Lilies open which are of course awesome but the whole downstairs is closed. The Cluny has about 3 or 4 rooms open along with the room where the Lady with the Unicorn tapestries are. Both of these are covered by the museum pass but they're not really providing your money's worth now as both have reduced entry fees.

On the other hand it makes more sense to do the Cluny and Orangerie on the Museum Pass than to pay full price for a ticket, if you can use the pass for a sufficient number of other site entries. I agree that the changes in the pass -- no entry to the Louvre without reservation and no multiple entries which was its major attraction for us, make it less valuable. We liked to be able to make a couple of short visits to major museums rather than doing a long all day marathon -- since that is no longer possible the Pass has lost its value for us.

My philosophy is that if a sightseeing card doesn't cover ones entire time in a city, the cost-effective approach is to use the card where your entry fee would otherwise cost the most per hour spent in the museum--assuming the logistics work out (meaning proximity of sights and alignment of opening hours). Unfortunately, it's not as simple as it sounds, because in a large city like Paris you can spend a lot of time zipping back and forth to sights, and if it's your first visit to a museum, how do you know how much time you will want to spend there?

I wouldn't automatically reject the idea of visiting a partially-closed museum during the validity period of a pass.

The Cluny has reduced entry fees right now. The Cluny is 5E. The Orangerie is 9E or you if you buy a combo ticket with the d'Orsay it's 18E. I don't have any old information on what the Orangerie usually is when the Walter-Guillaume Collection is open.