I've got an Olympus OM-1N...with no battery, of course. I've used the 1.4 Wein zinc-air replacement with satisfactory results. But I ran across
a place in California that's got the correct 1.35v Wein battery (also used in my
B&H Auto 35 and Minolta Hi-matic 7 rangefinder). A bit higher than it would have cost at Batteries Plus (but they didn't stock it) when you factor in Priority Mail, but the guy packed it in a HUGE ziplock bag (which I'll find some use for), and threw in some candy to boot! These batteries have been showing up on Ebay, too. So if you're got an older SLR laying about that uses the old mercury PX625 battery, the correct voltage replacement's now available.
Now, if I can only find a Zuiko 28mm f2.8 before this weekend...