Apple sells a Belkin KVM through its own store:
http://store.apple.com/1-800-MY-APPL....6.25.7.11.4.3

But why do you want to keep the outdated PS2 keyboard/mouse? Yes, you can pick up some PS/2 to USB adapters and they will probably work - but peripherals are cheap and you'd probably be better off finding a deal on a USB keyboard and mouse at your local technology shop.