I guess since you have a home built and not  a production aircraft you
hadn't noticed that Cessna switched to 28v in 1978.  My Piper was a 75
and was still 12v so I don't know if or when they made the switch.
I haven't found any problem with finding 28v parts.  Also  STEC
autopilots can be purchased in 14 or 28v systems.  Two of my airplanes
are 28V as was all the military and Nasa aircraft I ever worked on.  If
I'd gotten involved in the LNCE earlier I'd probably had that one as a
28V system too.  Even the UAVs I work on at NASA are 28v power systems.
They do also make 12v for any payloads that might need it.
duane 78FG177 79TR182 06LNCE  (Former 28v 78 182FG)