You used to have to buy the entire master cylinder when the lever broke. The dealers now should be able to get just the lever. I'm not aware of any aftermarket levers. CRG considered them at one point, but last I heard they were having a hard time keeping up with current supply demands.
There was a lever advertised on a German Web site, but it didn't look compatable to the stock MC to me.
I'd just count myself lucky that I don't have to buy the whole MC and visit a dealer.