try the air zinc variety, either a wein air cell (pricey) or a hearing aid battery (very cheap).
i'm using the wein only because i only recently found out about the hearing aid battery trick.
with the cheaper cell you will most likely need a rubber o ring to ensure a better fit. another trick is to get an old alkaline (625/675, can't remember offhand) button cell and remove the center part and insert the air cell into it.
i have also read, on karen nakamura (sp?) site that it's ok to use alkaline cells as the canonet is 'wired' as such that it will work just fine. (i can't recall the tech info)
joe