I'd check with Electric Motorsport http://www.electricmotorsport.com/store/ems_ev_parts.php
They used to sell those, so may have a source for the brushes. To replace the brushes, you need to remove the rear wheel and take apart the hub as far as I can remember. Those guys can probably tell you the procedure as well.
I'd suggest taking it apart and checking the brushes first before ordering. Brush dust has a tendency to build up inside and short the commutator segments. A good cleaning is all that is needed many times.