As far as I know, there aren't any retail-available internal hubs with disc brakes in the US. The SRAM i9 and Shimano Alfine are both available as (spendy) imports from Europe, though, and both can be had with discs. You could always use a front disc and a rear V-brake with a Shimano Nexus 8 redband, or get a nexus with a coaster or roller brake.