There is a website that you can use, but I haven't used it in quite a while so I can't confirm that it actually helps. Here is the link:
Location de véhicules utilitaires et voitures en France | E.Leclerc Location
I don't use the website because I have been renting from the same location for so long that I just contact them directly by email.
The rental process itself is fairly straightforward, but the paperwork differs significantly from what you might see at a Hertz, Avis etc. It is a little old-fashioned and cumbersome, so it takes a little longer when you are actually there to pick or return your car, but I just treat is as part of the adventure and allow extra time.
One quirk is that they can only rent the car for 30 days, so if you need it for longer, you have to go back on the 30th day and renew for another 30 days. The location I use is about 25 km from us, so we just use that as an excuse to go have lunch in Beziers.
Another quirk is the way they handle the payments. There are 2 separate charges: (i) a daily rental rate (5 euros/day for my car), which is paid up front; and (ii) a charge per km which is paid at the end. The charge per km has been creeping up for the last few years and now is around .25 euros per km, so it is not the bargain that it once was, but is still generally cheaper than other places, unless you plan to do a lot of driving.
In my experience, they don't have conveniently located rental agencies at airports or train stations, so the pick up and drop may involve an extra step in getting to the rental location. For me, that involves a taxi to and from the train station. Also, they do not allow after-hours drop off (or at least I have never done it), so if you are arriving or leaving outside of normal business hours, you will need a work-around.
I am happy to discuss this with you if you have more questions. We are back in the US for a while, so I should be fairly easy to reach. DM me if you want to discuss.
Ben