I found this:

One more IMPORTANT thing: I have completely solved the 625 mercury battery issue! Go to Home Depot, and buy a #9 rubber O-ring - It costs 29 cents, and then slip it in the empty battery compartment of your OM-1. You can then use the Zinc-Air 76-size batteries directly. These are the basis of the Wein cell, but they need not be bought at a high premium from limited channels like the Wein cell. They are hearing aid batteries, available at ANY Costco, Wall-mart, Radio Shack, etc. The voltage matches what the meter was built for, unlike any Alkaline or Silver Oxide replacements. So there is NO NEED to re-calibrate the meter. These batteries will wear out quickly (say, 1-2 months), just like the Wein Cell, but they cost 60-75 CENTS EACH in packs of 8 or more.

Can someone tell me which battery to look for?