Paris to Chartres is unreserved - no discount for buying ahead, and the tickets are good on any train (must validate before boarding).
Nice to Paris would have been much cheaper bought a few months ago. But looking now, I see that you don't save that much with advance purchase for June 26, so if you prefer to wait, you can. Note that one train for that day is already sold out (the 15:51 departure). So, if you have your heart set on a particular departure, you may want to buy it in advance.
To look at prices and buy tickets, you can use SNCF http://www.sncf.com/en/passengers (click "Reservations" on the left) or Captain Train https://www.captaintrain.com/en
Once in France, you can buy tickets at stations, but also at SNCF Boutiques, which are "train ticket stores." At the Boutique, you take a number (like in a bakery), and wait in the semi-air conditioning for your number to be called on the display. You then sit at a desk to complete your transaction. It's nicer than standing in lines at stations. They're all over Paris; it looks like the one in central Nice has closed, so the station is your best bet there.