Can the Museum Pass be purchased online? If so, does it cost more to purchase it on line. If not online,where is the best place to purchase the pass?
It can be, but depending on where you buy it, you'll pay shipping which isn't necessary. Most if not all of the museums included will sell it to you right at the museum. I'd wait until I get there.
Do a quick search on the "classic view" page (see search box in the upper right) and you'll see this very topic was discussed a couple of days ago, with loads of places listed to purchase it....including CDG.