Getting to Mont Saint-Michele from Paris via Rennes
There are a couple of ways to get to Mont St Michele form Paris. I took the Rennes route. You can take the TGV from Paris to Rennes.
Step 1. Take a train to Rennes
You can take the TGV from Paris to Rennes. The price varies but it takes just over 2 hours.
Step 2. Take a Bus to Mont Saint-Michele
The bus schedule to Mont Saint-Michele is designed to correspond to the TGV arrival times. Follow the signs to the Bus station. You'll want to exit on the north side of the station. Once on the street take a right and walk pas the construction. You'll see the bus station occupying the lower right corner. When I caught my bus I found the station wasn't open yet (Sunday). That's ok, the bus will still be there. Veer right and look for the bus to Mont St-Michele.
The ride is comfortable; on the outbound trip I had wifi, not so on the return trip.
You can buy a ticket right there on the bus, they have a credit card reader. They also take cash, but don't force them to make much change.
Heads up: There are only a few buses between Rennes and Mont St-Michele, so know which one you plan on returning on. The bus meets exactly where it dropped you off.