Hi RLO,
I'm unclear on whether you're asking if you can buy a ticket for
entry into Musée d'Orsay, or a
train ticket to the Musée. If it's a train ticket you're asking about, then yes, you can purchase a train ticket to get to the museum/art gallery, but it's not specifically just for Musée d'Orsay, it would be for anywhere within the Paris central region. If you're asking about tickets for entry into the Museum, no, I don't believe you can purchase such tickets at the airport.
The route is quite simple: take the RER B from CDG Terminal 2 to Paris St Michel Notre Dame station, transferring here to the RER C direction Versailles Rive Gauche or Pontoise or St Quentin-en-Yvelines. The next station on RER C will be Musée d'Orsay.
More detail on the
RER B from CDG Terminal 2 to central Paris.
It will take about 2 to 2.5 hours from the time your plane lands to arrive in central Paris including time for passport control, collecting your bags, walking to the train station, buying your tickets, traveling on the RER B into Paris.
Best regards,
Ben