My favorite daily-use mouse has always been the Microsoft Optical Mouse Blue which is USB and PS/2 compatible with the provided adapter. I have several of these mice, but the button switches have failed over the years on all but one of them. After a bit of searching, I was able to determine that the switch model is one of two models in the Panasonic EVQP0 Series of tactical switches. These two switch models differ only in the operating force necessary to activate the switch:

EVQP0E07K - 0.74 Newtons
EVQP0D07K - 1.30 Newtons

I don't have the equipment to determine the force required for the original switches, but after replacing the switches in one of the mice with the EVQP0D07K model and comparing to a mouse with the original switches, it appears the original switch is the EVQP0E07K with the lower operating force. So pick your poison. I prefer the stiffer switch. Both of the Panasonic switches are still available through a number of electronics distributors.

In order to get into the mouse, you will need to remove the four black rubber pads on the bottom with a small flat screwdriver or blade since these cover four screws.