You can just plug in both a PS/2 and a USB mouse and use either one without using a switch. There is no need to switch them, you just use one or the other, with most systems they will both work.
You could use a KVM switch and just use the M part to pick one mouse or the other, though you would need to use the button on the KVM switch to switch back and forth.
for a specific application package i just got i need to have 2 mice connected to my home pc. they must be manually switchable from one to the other. i thought this will be a no-brainer, but i guess i was wrong. all i can find is kvm switches. i want to plug in 2 (usb or ps/2) mice and flip a switch to decide which i will use.