www.mercotac.com
They make just about the only FIRST-legal slip ring that I know of. If you use your own brushings, expect to come under a LOT of scrutiny during inspection. The nice part about using COTS items is that the manufacturer spends a lot of time and effort guaranteeing certain performance metrics (in this case, voltage and current capacity). It is much harder to convince an inspector that your homemade brushes act in a safe and reliable manner.
FIRST doesn't fool around when it comes to the transmission of electricity throughout the robot...