I few minutes in google should provide plenty of answers. This topic has been beaten to death.
1)
PCI PS/2 cards are really USB cards with a PS/2 converter on it. You need to install the USB drivers for this to work. It will not work out-of-the-box with DOS. You will need to figure out how to get DOS to function with it, if it is even possible.
2)
There are dual-function PS/2 and serial protocol mice. The adapter used just swaps some pins around. The mice itself must be dual-function. The ones I've seen have been trackball mice.
3)
There exist protocol converters for translating PS/2 mice signals into serial signals which should work with most, or all, PS/2 mice. Some companies make them, such as Raritan (sp?) or Vetra systems, http://www.vetra.com/327text.html , however I found the conversion quality to be rather poor, resulting in a noticeable lagging or jumpy mouse at resolutions above 640x480. They are also costly, $60+. A user here made his own protocol converter, which you will need to package on your own. See this thread, PS/2 to Serial Mouse protocol converter
4)
Some serial and PS/2 KVM switches have protocol converters built-in. Cybex Switchview is one such unit. But again, I was not pleased with the tracking quality after conversion. Or was it the Belkin Omni View SE that had a working converter? I forget which of the two units it was, but one if them did not work at all, and the other I was not happy with the conversion quality.
5)
I have not played with the Microsoft bus Mouse 8-bit ISA card. You need a proprietary Microsoft bus mouse to use that. There may be some bus mouse converters to PS/2? PS/2 mice will not work on that port directly.
6)
I prefer the approach of modifying your computer's keyboard controller to work with a PS/2 mouse. It is a bit more involved, but there is no conversion quality issues with the mouse tracking. Refer to this link, Native PS/2 mouse implementation for 386/486 boards using the keyboard controller . To-date, only AWARD-based BIOSes seem to work with this modification.
Plan your life wisely, you'll be dead before you know it.