This question is like death and taxes. No matter how many times it is answered, it never goes away. So let's go around one more time!
Apart from finding an old mercury cell that still works, there are basically four options:
1. An MR9 adapter. This fits over a silver oxide cell and converts the physical size and voltage to the same as the original mercury cell. Apart from the cost of the adapter, this is the best solution. Silver oxide cells are widely available and last a long time. If you have multiple cameras that were designed for mercury cells, the adapter is well worth the investment.
2. The 675 hearing aid battery mentioned above. Once air is let into the cell, these discharge whether they are being used or not. So if you are not going to shoot a lot of film at one time, you'll need to have several of these as backups. Even with an O-ring, the fit in some battery compartments can be problematic.
3. The Wein cell. This is basically a zinc air battery that is made and marketed specifically to replace the old mercury cells. They have the same discharge drawback as the hearing aid cell.
4. Modify the meter circuit with a Schottky diode. Most places will do this as part of the CLA of a camera and you have to tell them specifically not to do it. I have several cameras where this was done and consider it somewhat of a pain, especially if you have multiple cameras. Unless you mark it somehow, you have to check the meter before using it to see if it has been modified or not. Also, the silver oxide cell is physically smaller than the original mercury cell, so there are the same fit issues as with the hearing aid cells.