I went to the famous Le Mont Saint Michel last week, and I wanted to share how to organize your trip! Hopefully it is useful!
If you are from Paris, you can go to Rennes first by French famous high speed train, TGV, by booking online through this site: voyages-sncf.com
Paris-Rennes ticket is usually cost 30 euro, unless in peak season it can reach 60 euro for one way. Therefore, book your train immediately! TGV offers ticket from 3 months ahead.
After that, you need to go out from Rennes Gare and heading to the left, to a building called ‘Gare Routiere’ or literally means bus station in France. You can buy ticket anytime in the ticket box, just mention you wanna go to Le Mont Saint Michel, they will give you ticket for the soonest bus. You can check the bus schedule here: http://www.destination-montsaintmichel.com/
Especially for people who lives in Rennes, you can get promotion price from October to March every year only by showing your Korrigo (bus) card only for 10 eur, because Fall and Winter season is off peak for Le Mont Saint Michel.