main attractions in Paris

Do NOT book from third parties. You'll pay more and you don't even know for sure if they're reputable.

Just wait. If you can't buy them until a certain date, they can't sell out before that date, can they? Look up when each site opens ticketing, put it on your calendar, and buy the tickets as they come up.

You can also investigate whether a Museum Pass will work for you.

Get a Museum Pass; we bought one last May and walked into all of these within about 2 minutes: the Louvre, Musee D'Orsay, St. Chappelle, l'Orangerie. We got a reservation for the Eiffel Tower, and our wait there was probably about 5 minutes.

I seem to recall that we couldn't get a reservation for the Louvre until just before we arrived in Paris, but the Museum Pass enabled us to jump all the lines and walk right in.

We had a reservation for the Eiffel Tower in April and waited 70 minutes in line to the elevator. Even reservations don't guarantee quick entry. Many places required and now suggest reservations even with the Museum Pass.

I ran into this trying to get a combo ticket for Sainte-Chappelle and Conciergerie.

Not available at the site website, for sale at tourist sites.
It turns out that the tourist site could sell the ticket, but not the time slot for Sainte-Chappelle.

I emailed the site and they said to book it one week ahead. This makes me nervous, but I will just wait.

We purchased the 4-day Museum Pass when we visited Paris. We walked right into the museums; did not have to wait in line at all. It is well worth it. I remember we purchased our museum pass at the Cluny Museum. It's a smaller museum, and the line was short. Of course, not sure what the lines would be like today at the Cluny. This was several years ago. We arrived right when they opened.

Covid has changed much of that; you need to have timeslots for multiple (but not all) museums https://www.parismuseumpass.fr/t-en/r%C3%89servation/cr%C3%89neaux-horaires

And with the post-Covid timed entries, some of the value of the Museum Pass is diminished. I'd not buy one unless you run the numbers and see if this is a good value for you based on what museums you want to visit. A lot of it's value was already diminished by Summer of 2019 when they changed the rules so you could only make one visit to each museum per Museum Pass. Yes, I used to pop in regularly to some of the smaller ones just for a comfort stop, lol. No more....

For myself, with having to make timed entries it's just as easy to buy single entries to the museums I want instead of the museum pass.

The only venue you really need to worry about selling out right away is Eiffel Tower if you want to go up. Whatever museums you want to visit you should be able to get tickets easily 3 months out. PROBABLY you can get them within a couple of weeks but that is a slight unknown at that time.

What museums do you have in mind? Maybe we can help ease your fears about getting in to specific venues.

BTW, don't be seduced by websites that say "skip the lines"....that is false. EVERYone stands in line for security which is the big slow down.

my usual note that you can buy lots of tickets at FNAC

Just backing up Pam and skunklet here.

The day of the Paris Museum Pass allowing direct entry to ALL of the museums it covers is long gone. We used this pass some years ago when it covered unlimited entry to those museums - we used it for 3 different visits to the Louvre during our week in the city! - and without any advance reservations. Now, you are limited to one visit per museum, and there is a list of those you are required to make advance, timed-entry reservations for; even the PMP website states "Remember to book your time slots in advance!" on the main page.

They provide a complete list of museums the pass covers, and notes which of those require "compulsory reservation of a time slot."
https://static.s123-cdn-static-d.com/uploads/3086630/normal_62569f23e04a1.pdf

And a bit more on this:
https://www.parismuseumpass.fr/t-en/rÉservation/crÉneaux-horaires

Kathryn, I just went back and looked at some of your other posts. I'd forgotten you are the poster who is organizing for a large group with 2 little kids. Is it 13 total with the kiddos?

Anyway, you've got time now, so I'd check each museum you plan to go to and see what the maximum amount of tickets you can buy at a time is. I am most familiar with the Louvre website and I tried to book multiple tickets. I could only book up to 6 at a time.

Not sure how many of your group will go to whatever museums you choose but I'd guess at least some will stay out with the kiddos or you will have split into smaller pods but you should still do some research on the maximum ticket purchase for each location.

I hope you have a wall sized calendar and giant spread sheet to keep track of all this, lol! And I hope your group is truly appreciative of your herculean effort.