The rule of buying in advance to save money is not only valid when booking a flight but also for train travel. Train tickets are generally on sale 6 months before the departure date. Some train booking tips from us:
There are a few train stations in Paris to start your route from including Paris Gare de Lyon, Paris Gare du Nord and Paris Gare de l'Est. In La Rochelle you have a few train station options to choose from for your arrival including La Rochelle and La Rochelle Porte Dauphine.
Omio customers traveling this route depart most frequently from Paris Gare de Lyon and arrive in La Rochelle.
Passengers board the train most frequently from Paris Gare de Lyon, which is located around 1.3 miles (2.1 km) away from the city center, and they get off the train at La Rochelle, located 0.4 miles (669 m) away from the city center.
Here are some other resources that might have the information you need