Guided Tour Ticket company

Always buy tickets to attractions from their official websites. A quick google search will show you most; here are the ones for those you mentioned:

https://www.notredamedeparis.fr/en/

https://www.sainte-chapelle.fr/

Many Seine river cruised to choose from but here are a couple:

https://www.bateaux-mouches.fr/en/cruise

https://www.bateauxparisiens.com/english.html

More and more, Viator comes up first on a search. Viator is basically a reseller of tickets and tours. I also recommend buying from the official site, not a third party that often sells the tickets at a premium. Sometimes the official website might be further done in a search. At this point, we've traveled a lot, yet there have been a couple times in which I couldn't figure out which website was official. In those cases, I got the official website out of the Rick Steves guidebook.

One of the problems with buying from resellers is that they often don't have the tickets. They sell the experience on the assumption they will be able to acquire tickets in time to fill the orders. There are a lot of people in Italy right now really ticked off that they will not get to tour the Colosseum or other important sites because their tickets from resellers got cancelled and it is too late to get them from the source.

Buy tickets from the site you want to visit.

And just to make sure you know....Notre Dame is free to enter. There are free timed entry tickets from the official website listed above but really the line moves fast so you do not need one. You DO need a timed entry if your goal is to climb the towers. There are also free tours of Notre Dame given by a cathedral volunteer group called CASA is you are interested I that information.

I agree with the others as well. Book directly and steer clear of ANY website that offers tickets across different venues.