Hi,
I've used an "old" serial Microsoft mouse, opened it, taken a cable from an old Amiga mouse and soldered the end of the cable at the right places in the Microsoft mouse -- bypassing the hardware that generates the serial protocol -- cut some traces and now I have a perfect Amiga (-only) mouse.
It's quite cool to connect a Microsoft mouse directly -- without any adapter -- to an Amiga :-D
It's easy to be done, just have a look into an opened Amiga mouse and an opened Microsoft mouse (two button) and if you have some electronic experience you could easily make the mod.
Noster