Hi,
If it's a real MP40 rather than a Modelgun, I'd suggest you check out firearms dealers in the US. I believe spare parts for real MP40s are made and sold by some specialist dealers.
Modelgun parts would be unsuitable as they're too weak to withstand pressures of live firing.
I'm sorry I couldn't be more help
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.