I replaced the battery in my Suunto Favor with a Maxell ER3S, a Lithium Thionyl 1/2 AA cell. It's the same battery that's used in Appel Macs. You can buy them from
Maplin for about a fiver each. (Search for ER3S)
Replacing it is a piece of piss. Even I managed it.
The electronics are all gel sealed so even if you do cock it up you just a need another battery.
Laters,
Janos