The number of reenactors that ditched the cups to fake a WW2 helmet is probably quite high. There's one on eBay right now with the remains of a LW decal on one side, bad repaint, and no chin cup. It's going to be tough to find a cup, but you do have pretty good luck finding rare bits and bobs so I'll put a fiver on you landing one
Cheers Steve and yes,thats the main problem in that reenactors back in the day before decent,cheap repros were available,used them and binned the chincups.