The SPI cable standard used on the system requires only 12 wires for 2 RHD chips (up to 64 channels), which makes it possible to build comparatively cheap commutator systems.
Adafruit commutator
Adafruit sells a chinese made 12-circuit slip-ring commutator for ~$25
We haven't tested these at all - but it seems possible that they will work, and if you decide to test them add your experience here!
Moflon commutators
these looks reasonable and have been tested with the RHA system (See comments), so they should work well with the current RHD chips.
Moflon Slip Rings (MC330; 18 leads; $65 - www.moflon.com)
They also make fiber optic commutators that look like they should work well for integrated optogenetic setups: http://www.moflon.com/mfo.html
It would also be pretty easy to mount 1-4 LEDs below the commutator and just route the LED drive currents through the slip rings, this could be easier in more demanding applications.
documentation for the Open Ephys Acquisition board has been moved to: https://open-ephys.github.io/acq-board-docs/