CT was running a "free batteries for life" coupon a while back - I have two of their grip sets on a pair of 1911's, and have yet not needed (it's about two years) to replace either battery set.
Radio Shack stocks 'em, too.... Not insanely expensive but free is better.... I need to find the paperwork 'cause it's about time....
The 1911 version uses a 3025 or something like that - a coin battery - and has no provision for recharging without taking the grips off the gun. This can't be good for the zero.... Better to run 'em into the ground and schedule a range session after the replacement (OK - check 'em regularly and replace when they start to go dim
.)
I don't know what's in the CT devices available for the M&P's, but it's got to be some sort of coin battery.... The rail-mounted lights, OTOH, could use just about any power source - more room
.
Regards,