Nowadays you won't get many PS2 keyboards. And many motherboards don't even have PS/2 port. Using USB-PS/2 isn't a solution.
This is actually not true. PS2 keyboards are good for gaming as they do not have the rollover problem which the USB keyboards have. That is you can press more numbers of keys simlutaneously without the keyboard blocking out then the USB keyboard ones. That is why the SteelSeries keyboards are usually PS2 and Razer tries to somehow reduce the rollover coz their keyboards need to be USB to support the glowing lights and stuff.
So if you want a cheap keyboard good for gaming definitely go for PS2. Motherboards will not be ending the support anytime soon and as far as I know, all motherboards have PS2 support. I have the latest Intel H77 chipset board. Even the Z77 boards have.