Let me start by confirming, do you have an older computer that takes a PS/2 keyboard & mouse, but has no USB port, that you need to use - or - do you have an older PS/2 keyboard & mouse that you want to use with a newer computer that doesn't have PS/2 ports?
USB ports have been around for over a decade now, and any system without a single USB port is probably near useless at this point, but if that is the situation, PS/2 keyboards & mice can be found used on ebay. If it's a PS/2 system with at least one USB port, you can hang a USB hub off that port and plug a USB keyboard & mouse in, pretty much every OS released in the last decade will recognize them.
If what you have is an older PS/2 keyboard & mouse, or maybe an older KVM switch, inexpensive adapters to connect those to a USB port are available.