How are you getting into Paris? If by RER it makes perfect sense because you can get the tickets on the way to the train. If using ground transportation it makes no sense because you would have to go down to the train and then go back up to ground transportation.

They're not generally on sale except at métro/RER stations, so if you're not taking public transport into Paris, no, I doubt it.

Don't understand what would be such "tight connections" upon returning from Normandy to Paris. If you have to take a métro to get to your hotel, that's when you buy the tickets.

The RER is officially an extension of the SNCF system, so I doubt that they will sell a carnet for the métro even though you can use the métro tickets on the RER within Paris and the RER ticket looks like a métro ticket. What you wrote seems to indicate that you are by-passing Paris on your way to Normandy, hoping to have the métro tickets upon your return to Paris. The lines are so long at the ticket counter that you do not want to buy tickets unless absolutely necessary. If picking up a car at CDG, go directly to the rental agency and forget about métro tickets. If you are going to Paris, your RER ticket is valid for the métro.

Métro tickets (tickets t+) are sold at tabacs as well as RER/métro stations. I have never actually looked for them at CDG but I can´t imagine you could not purchase them there, particularly at either of the RER stations T1 or T2. You can buy them at St Pancras in London so I don´t know why you couldn´t find them at CDG. You just can´t actually use them until you are in Paris.

Thanks for all the replies. Yes, I am bypassing Paris after landing at CDG. I already have car rental for Normandy and dropping it off at Caen and taking train back to Paris. I have purchased the RER tickets already for return to Paris. I was just trying to plan ahead so that I would not have to buy Metro tickets when arriving at St Lazare. I will just plan to purchase after arriving in Paris. Thanks for all the comments. It is appreciated!

When you exit the train at St. Lazare, you'll see an exit to your left, take it and it'll lead you outside. You'll have a few stairs to deal with, but the entrance to the metro station is right there. Enter the station and you'll find at least two ticket windows to purchase the tickets you need that are easy to access. The windows are next to the two different entrances (each takes you to a different platform area - read signs for yours) so go to the window closest to your platform. We bought Navigo Cards at these windows after our return from Caen in March 2011.

It doesn't make any sense to try to buy them at CDG and keep track of them while you are in another country instead of buying them when you actually need them. I don't get it, why would it be easier to buy them at CDG than at a station in Paris. It only takes a couple minutes anyway, you can buy them in the machines. I don't know why people make such a big deal about buying metro tickets in Paris.

Thanks apersuader65 for the tips! That is very helpful. Since I am doing all the planning for this trip all around France and have never visited Paris, it is nice to have good directions like yours!! I really appreciate it!