where I can find source code of cv2.BackgroundSubtractorMOG?

edit

Well, originally both BGSG methods where in the main branch, however MOG2 outperforms the MOG version by far, and thus there was a decision to keep only MOG2 in the main repository right here inside the video module for the master branch.

However the MOG implementation can still be found in the contrib repository in the bgsegm module, right here. Build the repo with the contrib modules and you will be able to access it.