I am planning on traveling from Paris Quartier latin area to Disney in June and was trying to buy train tickets but was only able to find 1 month on the calendar . are tickets not available for advance purchase ?
If you're looking for RER train tickets, for the RER A line, say from Chatelet les Halles to Marne la Vallée (where Disney is located), there are no reservations at all. All RER trains are unreserved/open seating and the tickets (5€, one-way for Paris - Marne la Vallée) are good for anytime in the future. (You could buy a RER A ticket for this Paris - Marne la Vallée one day and use it the following week, or following year, etc.)
A single RER ticket is also good for transfers on the Paris Metro to get you to an RER station to make your trip out to Disneyland on the RER A. So for example if you're starting in Quartier Latin you could take Metro 10 to Metro 4 to get to Chatelet les Halles, then take the RER A to Marne la Vallée. That entire trip, on both RER & Metro, you could use a single RER ticket (for "Paris to Marne la Vallée", 5€) to complete.