We are traveling from Paris to Nice by train in September. Where do you book tickets?
Tickets may not be on sale for September yet. Used to be a 90-day limit; however, the operator now sometimes runs advance promotions. Go to the website and sign up for e-mail notifications. Booking as far ahead as possible can save lots of money.
You can book them on the official site SNCF https://www.sncf.com/en although there are alternatives that work just as well or better. I like https://loco2.com but there are others as well.
This site has a lot of really good train travel information for all over Europe: https://www.seat61.com/international-trains/trains-from-Paris.htm#Paris-Nice
There are several types of trains at different speeds and price options so explore and understand your options.
Have a good trip,
=Tod