For an MP40 AEG AGM is the main way to go. Cheap and cheerful but don't expect anything too amazing,AGM QC is pretty bad. Mix of steel and pot metal but thankfully no pot metal placed in strategically wrong places.
For a GBB Marushin make one but it's expensive and fires 8mm BBs so more expensive ammo. It's a standard marushin gun,okay kick,meh accuracy,more a collector item or backyard plinker.