There are two way to find them. Through your agent card in which you may be lucky and find a legendary Pokemon or through Multiplayer (Battle/Pair). I was lucky enough and had Latios on my Agent Card. Also, I was once multiplaying and found Groudon... But my evil friend closed his DS and I didn't catch it... Ah well...
i 've not seen any legendaries, and i'm the only one with link (thats wat its called inn england :/) around where i live, + i was unlucky engough to get venusaur on my agent card when i already knew where itb was.