The opening sequence of The Dark Knight is the Joker's thugs robbing a mob bank. At the end of Batman Begins, Batman is asked by Gordon to help track down someone who has been robbing banks, and Gordon then gives Batman a joker card. It is assumed that between Batman Begins and The Dark Knight that Joker had been robbing mob banks, which is how Joker was able to afford his purple suit (he says something along the lines of "Like this suit? Well 'cause you guys paid for it," then laughs).
