In summary, there are three options:
1) A zinc-air hearing aid "battery" (actually a cell). These have a limited life once they are exposed to the air. If you are going on vacation and you are going to shoot a lot of film in a week or two, these should work fine. Otherwise they will likely be dead the next time you pick up the camera. Wein cells are the same thing; they just get to charge more money because they are serving a "niche" market.
2) An MR-9 adapter. This is a shell that fits over a silver oxide cell and converts both the voltage and physical dimensions to match the mercury cell the camera was originally designed for. They can be used in any camera and cost as much as 2 -3 rolls of film with processing. One will probably last forever.
3) Have the camera modified to use a silver oxide cell. This involves adding a diode to the meter circuit to adjust the higher voltage of the silver cell. John Goodman includes this as a standard part of any CLA (i.e. you have to tell him not to do it if you want to use one of the alternatives above). I would imagine that any reputable repairman would at least offer to do it at a nominal cost. Since the silver cell has a smaller diameter than the mercury cell, this requires an O-ring or similar spacer to center it properly.
If you intend to mix methods (e.g. you might want to keep some cameras "original"), you need to be careful to check which cell to use before loading the film. Never use an alkaline cell in these cameras. these have a non-linear voltage/discharge curve.
