Suns do not use PS/2 keyboards and mice. The adapter is the only way to go. Of course, newer Suns use USB for keyboards and mice, so if you're getting new equipment, you should be okay there. Sun has a list of verified-compatible third-party equipment you might want to take a look at. (Sorry. It used to be easier to look at.)

Of course, this all begs the question of why your Unix consoles are graphical and not text-based serial, which are much more accessible.