Train tickets

Can you buy tickets for the train from Gare Saint Lazare to Vernon at the RER station at the Musee D'Orsay? I know that the RER is different from the SNCF train but wondered if they sell the tickets there. I will be traveling from CDG into Paris via the RER to the Musee D'Orsay stop to spend the night nearby and would like to purchase the other train tickets ahead because I will be leaving early morning the next day and am thinking there will may be a long line at the station and don't want to have to get there super early.
Thanks

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