Michelin and IGN maps are widely available in highway rest stops (autoroutes and some routes nationales), larger tabacs in towns and cities, bookstores (the French word is librairies), and hypermarchés (large supermarkets such as, in the the U.S., Fred Meyer, Meijer, and Walmart Supercenter) -- these sport names such as Carrefour, Hyper U, E. Leclerc, Géant, Auchan, and Intermarché. Their locations are usually well-marked by directional signs posted on the outskirts of small- to medium-sized towns (they're in big cities too, but harder to find and traffic can quickly become difficult).
It would take very little time or effort to obtain them in France after you arrive, and you could use the same stop to pick up some essentials such as water, snacks, fruit, toiletries too big to fit in your carryon bag, and so on.