I think one of the masters of yakuza movies is Takeshi Kitano, you can Watch from this director :
"Kid's return", a story about two hightschool friend who prefer truancy, one want become a professional boxer but the other is going to integrate the mafia. A touchant story, with a lot of nostalgy.
"Zatoichi", the plot takes place in Japan during 19e s, it's the story about a blind warrior who fight against a gang, a virtuous movie which associate sword scene and humor.
"Hana-Bi", A story about a policeman who quit police for dedicate himself to his sick wife and made an hold-up. A poetic masterpiece.
And he made a lot of other yakuza movies, these are all really good but some people find them two much slow. I hope you enjoy, you must give a try ;)