I got my spare battery on Amazon, have had no problems with fit. I got an Anker 1600mAh battery, specifically made and labelled for LG Optimus V / VM670 / Optimus M / MS60 Optimus U / OPtimus T // Optimus One / P500. Apparently they all use the same size battery.
When I bought mine, I had the option of buying the battery or, for the same price, buying the battery and an external charger. No-brainer for me there. Only 100 more mAh, but it seems to last noticeably longer. YMMV, of course.
OT: My battery life on CM9 is improving, now that I am not changing things around as much, currently at 65% after 10 hrs (old battery, should get over 24hrs on the Anker).