A lot of those type of carb “hard parts” for Holley carbs have never been available to the general public.

Just try a few different established carb builders, they can usually get those type of parts.

I used to buy boosters from BLP.

68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads