Great question — answering as the UphillTech team:
What it comes down down to is finding an operating system that supports the processor architecture in those machines. Sun machines generally uses the sparc family and for FreeBSD there are releases that supports this. http://www.freebsd.org/where.html#download contains links to isos for the various platforms FreeBSD supports, including sparc64 which would be the one you should download for installation on your machines.
Full answer by stralle: https://superuser.stackexchange.com/a/508610 (CC BY-SA 4.0).
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