You can get some tickets online via the official website, although when I last checked there weren't many left for the main courts (only one day for Philippe Chatrier/Suzanne Lenglen and two days for Court 1, all in the first week). Plenty of ground passes available, although you should be able to get them easily on the day.
And when I went back in 2007, people do ask you for your main court(s) tickets on the way out
Makes sense, otherwise they're just resold to someone else.