The Booster Deadline is January 15th
One of the great things about living in France is the seriousness with which they take Covid. As soon as Omicron was announced they took no time in authorizing boosters, and issuing a public decree that all boosters must be taken by January 15th or your health pass would expire. If it has been 7 months since your last dose, your health pass would expire before January 15th regardless. If your health pass expires then you will no longer be allowed into public spaces.
Of course this led to a mass rush to grab available appointments. During the first day people were waiting 3-4 hours to make them online, but now it has worked itself out, and there are plenty of vaccination sites sans RDV if you fail to nab an appointment. If you are still unable to find one, note that nurses and pharmacists are allowed to administer the shots. I got mine in the US so it was free, but if you decide to get it done in France, and you do not have a carte vitale, it will cost 8€.
As most people are going on holidays, some have asked what to do if they get the shot outside of the EU. In this case you would come back to France and see a participating pharmacy which can be found here: https://www.sante.fr/obtenir-un-passe-sanitaire-en-cas-de-vaccination-letranger. The cost of translating your booster into the QR code will be up to 34€. A scam when it used to be free, but what else can you do?
For those crazy enough to choose the antigen/PCR option, note that it is only valid for 24 hours. PCR tests in Paris are 44€, or free if you have a carte vitale. Why you would prefer to have you nose swabbed every-time you need to go to a public space instead of just taking a simple shot is beyond my comprehension, but this will change from the 72 hour validity it used to be.