Other things to consider are:
External or panel mounted (interlocked?)
How often it will be actuated
Compliance with Lockout/Tagout
Enclosure type (suited for environment)
Susceptibility to damage (believe me, it will happen)
Which Mitsubishi PLC are you using?
This will work for the FX series.
Search Newark for a 3.6v lithium battery. You should be able to find one with solder tabs. Use your existing lead on the new battery.
http://www.newark.com/
Change the battery with power on to the PLC. If the PLC does not...