Yeah, it is very difficult to source 18mm screw hole spacing M12 lens holders. And the ones that I've found are usually plastic rather than metal and sometimes not available in small quantities. With the Mobius, if you need to replace the lens holder and want a metal one it often comes down to whether you are willing to purchase a new module. Then again, the fact that you can actually buy a replacement module that you can easily swap in is quite remarkable compared to virtually any other camera on the market. One the other hand, if you feel a plastic one might work for you, you can source them
HERE (AliExpress) Some of the plastic ones you will find are made of FRP (fiber reinforced plastic) which is dimensionally stable and therefore a reasonable alternative to metal.