Post Office Changes currency and has an ATM. It's not far from your point of entry on Mont St-Michel, Porte de l’Avancée.
La Caserne Tourist Office Just before you approach the bridge, this is a handy port of call for brochures and info, with left-luggage lockers and an ATM.
Mont St-Michel Tourist Office Just as you enter Porte de l’Avancée, this place is up to your left. You can check the tide tables here, change money and buy a map of the Mont (€3). Next door are toilets and an ATM.