Yesterday we went to the Louvre because it was a train strike day and so we postponed going to Meaux for the WWI museum. It was a rainy day usually a day the museums tend to crowd up. We walked through the security at the Carrousel with no line at all. In the ticket room we waited for about 5 minutes and bought tickets from a clerk; there are 8 or 10 ticket machines in the room and 8 manned windows. We could have bought a museum pass right there without paying extra for shipping or worrying about getting it in time. You can also buy them at the airport tourist booth on arrival.
The ND can only be purchased from a local train station; you can get them in the metro which we did this year as our old ones had expired or at an RER station at the airport or elsewhere. Be sure you have assembled them and printed your name and affixed your picture before use.
We have had our passes checked twice in random checks on the metro since we got here.