Yes you could do this, from Sun Microelectronics back in the 1990's.
We purchased an Ultra-10 motherboard with 250 MHz Ultrasparc II in 1998/1999, fitted it out with compatible third party SVGA, Ethernet and SCSI adapters and ran Solaris on it. Was our production LDAP, Mail and database server for a small ISP for many years.
I later worked for a systems builder that built and sold Sparc/Ultrasparc systems for government (eg. Hong Kong Mass Transit). We were the only vendor in the Southern Hemisphere doing it. So it wasn't common. But yes you could order parts from Sun Microelectronics and build your own systems, and it became easier use certain standard PC compatible parts once they standardised on PCI. We had shelves full of the older incompatible MBus compatible systems and boards as well.
I later worked for a systems builder that built and sold Sparc/Ultrasparc systems for government (eg. Hong Kong Mass Transit). We were the only vendor in the Southern Hemisphere doing it. So it wasn't common. But yes you could order parts from Sun Microelectronics and build your own systems, and it became easier use certain standard PC compatible parts once they standardised on PCI. We had shelves full of the older incompatible MBus compatible systems and boards as well.