You can buy it through Rail Europe or www.voyages-sncf.com does offer an option to stay on that site. You may have to say that you live in France or will pick up tickets in France. (They'll be e-tickets.) You can later change the language to English.
[edited] For a route that's primarily by train, start by requesting a schedule to Pontorson, since Rennes is not the only possible connection point, and some schedule results will show both legs by train. For instance, the 10:10 Mon-Fri departure connects at Lison and some weekend schedules connect elsewhere. However, it's too soon to book or see confirmed schedules for the regional trains on this route for your travel dates. You could try again around October 1.
Or, searching to Mont St Michel will give you different schedule results, usually including a bus connection at Rennes that goes all the way to MSM instead of just Pontorson. If you prefer those, they are for sale now. The portion by TGV to Rennes is the more expensive and reserved leg of the trip, with the greatest advance-purchase discount, but you can purchase the bus connection as part of the same ticket.