McMaster-Carr
Grainger
Fastenal
If you strike out on all of these, you may have to source it from the manufacturer. (Ktm motorcycles use this size)
If that doesn't work, recutting threads on a m10x1.5, Preferably a very long bolt with a large shaft to re thread. Alternatively, drilling and retapping the hole.
Definitely use anti seize compound on this bolt. Fine threads mean more surface area, means more friction, means more torque required to break it loose once it is seized...... And as you are limited by the Allen key, which is why counter sunk heads are typically course thread only.