Agreed that alkaline cells are not suitable. Silver oxide have much more stable voltage over their life and I have found that the above works, well enough at least, in sunlight and for very low light shots.
1.4v Zinc Air (Wein-type) batteries can be bought at Boots the chemist - I picked up a six-pack recently for £3.75. Look for Boots 675ZA batteries.
It's also worth keeping an eye on Ebuy - I managed to grab a 60-pack of 675 batteries (dated 2013) for £2 including postage :D
As I wrote before, one alternative is to go to www.zuiko.com and buy the MR-9 adapter, into which you insert a silver oxide 386 battery and then put the MR-9 into the battery compartment; it fits the spot any PX625 or PX13 would otherwise fit.
If anyone has a Canon EF although originally designed for 1.35 V Mercury cells you can quite happily use 1.5V alkaline or 1.55V Silver Oxide button cells in it because it has a self regulating circuit that will accommodate the voltage difference, but not batteries with rubber "O" rings round them because they need to make electrical contact all the way round.
another option which will work without modification is a VARTA V625U 1.35, but, it is alkaline and you need to check/change often, i have an SR7 and use these batteries.
See at www.smallbattery.company.org.uk based in London, they have the best and large options to replace the old outlawed PX625 1.35 mercury batteries, PX400, PX32, PX675, PX27 ecc ecc. Personally, i have bought some Kanto MR9 adapters to replace the PX625.