There are 2 ways.
You gamble and buy luxeons of unknown bin and hope you get lucky. Very lucky, since there are few outlets for known-bin Luxeons.
You buy from a source that will sell known bins. You can generally only do this in small quantities (1-10 pieces) or huge quantities (thousands of pieces... per month).
There are occasional group buys in the Group Buy forum where the bin is specified. Some of the "boutique" retailers sell binned luxeons.
There are also individual sales of binned luxeons in the B/S/T Lights forum - some from individuals with regular sources of binned luxeons, others from people that gambled on quantity and are selling off extras or bins they don't want.