For some already existing info/advice, use the Search feature here to search on French Open and take a look at my posts about getting tickets for last year's F.O. We used the official F.O. reseller site (http://www.viagogo.fr/rolandgarros
) and it worked very well for getting stadium tix. You can even put your grounds passes up for sale there as well, if you get stadium tix.
My advice - stay on the two larger stadiums (Chatrier or Lenglen)...while Court 1 (aka "the Bull Ring") is a great fan court, the quality of the matches on it last year was disappointing, we would have preferred to have seen the better matches on the other two stadiums, even though they are larger.