You wouldn't expect to find a store dedicated to selling rain gear where there is insufficient demand. Rennes does get plenty of rain, with September and October being the heaviest months for precipitation. Though Rennes doesn't get as much rain as western Brittany, it must obviously get enough.
My experience was that it would rain, sometimes real downpours, but these tended to be fairly brief.
There is all sort of umbrellas from the silly to the elegant and functional as well as rain jackets and ponchos and all sorts of rain gear. If you get stuck in a non stop downpour in Rennes for a few days, this is the place to go!
Address: 27 boul. de la Liberte Rennes
This large shopping centre is near the railway station.,and museum. Many shops. Centre in city centre.