The sites iFixit repair guides for Mac, &/or the site powerbookmedic may have some info on the parts.
Sometimes these kind of sites have reference to some part; while on rarer occasion may sell them too.
Other vintage mac repairers, and recyclers may have access to these rarer parts; a few may do rebuilds.
{Some time ago take-apart/upgrade views were available from http://www.xlr8yourmac.com/
however as time goes forward, those unique bits of information don't get carried along.}
There was more than one build model version of the G4 Cube, so part numbers & availability may vary.
• Power Mac G4 Cube Repair - iFixit
www.powerbookmedic.com/Power-Mac-G4-Cube-Power-Button-Board-p-24824.html
service parts G4 Cube(?)
http://www.powerbookmedic.com/xcart1/home.php?cat=745
https://everymac.com/systems/apple/powermac_g4/specs/powermac_...
https://everymac.com/systems/apple/powermac_g4/specs/powermac_...
http://www.usedmac.com/products/apple-powermac-g4/powermac-g4-450mhz-cube-384mb- 40gb-dvdrom-pre-owned
..Several years ago these were more frequently found as parts or whole machines..
Not sure what you'll find now; regional sources for vintage Macs & parts may be best.
