Try "The usual suspects" (call them they frequently have lots of stuff in stock that isn't listed).
Springfield Sporters Inc:
Numrich Arms, aka E-gunparts.com:
Make sure you specify "for the #5 Jungle Carbine with the 800 yd maximum setting
", the #4 one that goes to 1300 yds is not correct. If you get a stamped one get the spacer as well because you'll need it.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”