If they haven't changed lately, SNCF tickets (on their sites Ouigo and SNCF-Connect, not very user-friendly unless it has changed lately; and probably on reselling sites) are usually cheaper shortly after the sales opening than near the journey, on a non- or partly-refundable basis.
Here are the sales opening dates depending on the type of train you wish to use:
SNCF Connect : L'ouverture des ventes - Aide et Informations
www.sncf-connect.com
In any case, if you're landing in Paris, you'll have regional trains between Paris Montparnasse and Angers, they're nearly never full and rarely at reduced prices (and their sales start three months before the journey), actually many travellers just buy the tickets when they arrive at the departure station (but there may be queues at both automated and in-person selling points). Paris subway isn't very pleasant though, nor very safe and there aren't escalators or lifts everywhere. You can already calculate your journey with them because they won't change their hours between late September and October.