Looking for battery for old microphone.

how old is it?
how big is the battery compartment?

Take it to Radio Shack, they have many of the more obscure batteries.

I was just there a few days ago doing that myself for the keyless entry fob for my car. After nearly 18 years of use the battery in it finally died while I was out shopping and couldn't lock the car door (the key slot had long ago frozen shut). They also had a teeny tiny battery we needed for a remote control for light switches that no one else seemed to have.