I am due to arrive in Paris in two weeks time. I have tried to book a day pass for the RER online however it won't deliver to my hotel (near CDG airport).
Can I buy the day pass at the ticket machine on the day?
Ticket Mobilis day pass is available from every Metro/RER ticket machine.
Note that Mobilis is not valid for the RER B to/from CDG Airport. You can take the city buses from CDG to Paris though, as long as your Mobilis is good for 1-5 zones. If you're not doing this trip multiple times in one day, I suggest getting a single-use RER ticket for getting into Paris, then using a Mobilis 1-2 zone for all your trips around Paris city center.