I was surprised to see this offering, a machined M12 lens holder than is designed specifically for the R-Pi camera board:
http://www.m12lenses.com/CNC-Machined-R ... 020rpm.htm
I'm not sure if this is a good idea, given the standard RPi camera module is not guaranteed to be strictly co-planar with the PCB (giving you a somewhat varying focal distance across the surface of the sensor). At any rate, it's a more convenient alternative to carving out a stock M12 holder to mount on the board, which I've done before, and allows use of an M12 lens with the official stock camera board, instead of being at the mercy of the aftermarket M12 board suppliers. Note also, not all M12 lenses will work; some have a back-focus distance too short and mechanically interfere with the plastic housing of the sensor before they are threaded in far enough to reach focus.
I'm not affiliated with this vendor. I have some purchased lenses from them, which were of the quality expected from the price.