Yes you can. The RATP Website has a list of designated official agents. All tickets and passes are easily availble in Paris, at metros, tourist information centers, etc. The Carte Intermus e pass is also available at all participating museums. There is usually no line.
Passes purchased through agents usually have a substantial handling charge added on.
There are a couple of options available to visitors to Paris. The most expensive is the taxi. There is also a 6FRF charge for each piece of luggage. A cheaper option is to contact an airport shuttle service. The going rate is approximately 180FRF for 2 persons for door to door service, 120FRF for one person.