take a look here if this interests you: PS/2 to Microsoft Serialmouse Adapter Converter / Updated First Post / Firmware Update added

I went through this about a year ago with my 486 and settled on the Logitech MouseMan Serial (the triangle shaped one). It comes with a PS/2 connector but is designed to work with serial adapters. It's the most comfortable I found, with very light button clicks, so no finger fatigue from hard-to-press mouse buttons (which a lot of older mice were guilty of). Here's one:
http://www.ebay.com/itm/Logitech-MouseMan-Ser … %3D201862024029

Not new, but it gives you an idea.

Thanks. I think I have used one of them before. I have a box of old serial mice, but they are all quite well worn.

Different PS/2-serial adapters will work with different mice.

I have a green Logitech adapter that works fine with my Logitech mouse, but doesn't work with my Microsoft mouse, and I have a Microsoft adapter that works with my Microsoft mouse, but not the Logitech mouse.

I guess there is no standard and they are wired differently.

Also keep checking thrift stores, online classifieds for your area, and potentially (if you have lots of time), garage sales. Lots of things show up.

I've currently got a Kensington Expert Mouse 2.0 that a friend found at a garage sale along with about 4 more of them. Kinda weird to find that many, but it's a damn fine trackball, so I'm not complaining.

All of the mice I have in a box are from thrift stores. They are all a bit worn though, I would like something new.