Sourcing in Europe:
- DIY stores
- nearby mechanic - I am sure if you ask nicely they will help
- internet stores
Just google "screw M10x12".
If you cannot find exact length and you decide to cut it, put the nut on above the cutting place, so when you screw the nut out after cutting it will shape the thread. If it is a steel screw you may put some paint on the cut end to prevent rusting.
I would probably consider a stainless steel one, but that is only me.