hi there now that mercury based batteries are banned from most countries,
i have got one of my camera repair buddies to make some little blue rubber rings to hold hearing aid batteries in the cameras that take the 400px? battery
you can buy the expensive wein air cells, but the hearing batteries of the correct voltage are NZ 10 for a pack of ten batteries, or about US $4.00
i have a few spares if any one wants a couple, i am in New Zealand at firstname.lastname@example.org. they are free all i ask is a self returned stamp enevlope that i can send them back where ever
i have about 10 spare so at 2 each thats 5 spotmatic fans
Now to my understanding after some research through a few other Pentax Spotmatic groups, the circuit used with the meter can tolerate a 1.55 volt battery. This is consulting a few groups on Flickr and a Pentax forum somewhere.