We just got back from a week in Paris, and the COVID test requirement was one of our biggest worries. It turned out to be very simple, very fast, and very convenient.
We were in the 7th not too far from Rue Cler, and there are pharmacies all over. Many (most if not all) have signs in French indicating whether or not they are performing COVID testing. Also, there is generally further information on the type of test and the turn-around time.
My wife walked in to the nearest one and asked the pharmacist, and was told they did not do the testing, but the one a block down Rue Bosquet did, so we dropped in to that one, she spoke to one of their pharmacists (in English mostly), and he explained the process.
Here is what it was for us:
1) Scan a QR code that directs to a website (Internet access required);
2) Fill out the fields on the website and receive a confirmation number to show the pharmacist;
3) Get in line at the pharmacy (5 mins), show the pharmacist the confirmation number, and indicate you want the rapid antigen test;
4) Pay ~29 Euros (if not covered by French healthcare) and receive a receipt to hand to the test personnel;
5) In our case, go outside to the test tent, hand the tech the receipt, get swabbed, and then go about your day;
6) 30 minutes later (for antigen testing), check your e-mail and see the results.
As it was a PDF result file, I just then later submitted it through the United "Travel Ready" site, and we were good to go a couple days later for our flight home.
In other words, it was super-easy to do and should not be a worry if you are in a city like Paris (and likely most medium towns).