Hmmm... it's kind of tough this time of the year, for RG you have to plan in advance, ask for a ticket form and fill it, send it with your money and then hope you can get some tickets, I did that in January, and I still don't know if I got any tickets.
You can always buy them from the scalpers. There are PLENTY around RG. It is actually a bit shameful, since they are re-selling the tickets at 5x the price (or more) in front of the police. You can get Annexes tickets (all but the main courts) fairly cheap though if you speak a bit of french.
Whenever I had to buy a ticket from the scalpers I had only bought Annex tickets (the other are sold at very high prices), you just wander around until you find somebody willing to sell for a reasonable price. If you are asking for Annex saying no to high prices, adding "Rien ā voir" usually does wonders. They don't realise you are probably more interested in watching those little known players anyway