dnd 5e – Are there any existing magical/artificial replacement arms written up?

Yes. Man, are you in luck.

The DMG details two such replacement limbs as magical artifacts- The Hand and The Eye of Vecna. Both of these items replace original body parts, and grant a number of randomly determined enhancements (20 STR score or permanent True Sight, for example), as well as an Evil alignment. The Hand specifically call for the intended user to lop off their own left hand, which might fit your specific circumstances (and depend on just how much arm was lost).

Obviously, as artifacts of an undead god of secrets and magic, these are tremendously powerful, and you should consult your DM before attempting to quest for them.