I saw this mentioned in a reply on another thread, but I wanted to emphasize that while walk-in Covid testing is widely available in Paris, this isn't necessarily the case elsewhere in France. You will likely need to make an appointment via the Doctolib website (which unfortunately seems to crash a lot on my computer).
In my experience in Brittany and Normandy, walk-in appointments are not readily available, and even same-day appointments seem to be very limited. Also note that many pharmacies offer little or no testing on weekends, so people flying to the US on Mondays have little flexibility on timing.
There is a Carrefour in Bayeux that offers both appointments and walk-in testing. This is where I ended up making my testing appointment. I will update this post if I run in to any issues there.
In hindsight, I would have purchased the BinaxNOW tests before my trip had I known about the testing situation outside of Paris. I was scared off by the lengthy wait times I read about, but those issues seem to have been resolved now.