I am also looking for an alternative battery for my M12 - either LiPo or LiFe packs. That Protek LiPo Tx batt is always back ordered online.
Found this one. Dimension wise it should be ok to fit in the battery compartment of the M12
82 x 30 x 18 mm / 3.22 x 1.18 x 0.71 in (Protek's dim = 99.2x31x17.9mm)
A little bit less in charge capacity, but $5 cheaper though.
Venom Power 5C 2S LiPo Receiver/Transmitter Flat Battery (7.4V/2400mAh)