You have a few options:-
1. If you shoot negatives, then just use the cheapest unstable voltage lithium equivalent (LR9/V625U
). Negative film and the printing process has a wide exposure latitude and so most users won't notice any fault.
2. Use the silver oxide equivalent (S625PX
) which has a stable output voltage of 1.55v which is higher than the 1.35v original. Again you can either rely on exposure latitude, or if you shoot transparencies you have the additional option of compensating by up to one or two stops as necessary by adjusting the film speed dial. As the output voltage is constant, so is the compensation for the life of the battery. Or you can get a camera technician to recalibrate the meter to suit the higher voltage. Compensating has a downside though and it's that you're effectively shifting the EV scale so you may lose some of the exposure range which your meter can cope with.
3. Use a Weincell MR625
Zinc/Air battery. This has the correct voltage but they don't last as long and cost more than the other batteries listed so far.
4. Use a CrisCam MR9 adaptor
. This is a one off purchase of a voltage reducing adaptor which houses a silver oxide cell. You can move it from camera to camera, and as the voltage has been reduced rather than compensated for you retain the full EV range of your meter. If you live in North America it's cheaper to buy directly from the US manufacturer, but they charge really silly money for shipping overseas.
5. Although I have an MR9 adaptor, I've switched to using an excellent and technically superior alternative which is custom made to order by Frans De Gruitjer in the Netherlands. It's about half the price of the MR9 adaptor, has superior output characteristics which mean it's also suited to cameras with a high current drain. Or you can make your own, from scratch, using very comprehensive instructions
he provides or make one using a kit he provides. If you buy them ready-built, then postage is included if ordering within Europe and they have a five year warranty. For current details on prices and availability, contact Frans at email@example.com . There's no language barrier as he speaks perfect English. Unlike the CrisCam unit the adapter is supplied without a battery but you can obtain the correct SR44
's easily elsewhere. Frans also allows purchasers three months to return the adapter for a refund for any reason at all.
Please note that many of the links are to a UK battery supplier. I'm not connected with them in any way, but if you're in the UK/Europe I highly recommend them. However, my reason for linking to them is solely for the comprehensive information they provide.
Good luck whatever you decide, and I trust that you will return the favour by taking a brief moment to rate my answer.