Factory brake booster question

It kind of depends on which booster you have; in '67 there was a 6 1/4" Midland/Ross booster, and a 7 1/2" Midland/Ross booster for A bodies. I believe the smaller one was used on big blocks, and the other on small blocks and slants.
If you have the 7 1/2" booster, great. Rebuild kits are readily available from classic Mustang suppliers, since the same booster was used on Mustangs and Falcons, among others. The main difference is in the front housing, which is stamped for either the Mopar four-bolt master, or the Ford two-bolt master; and then the appropriate filter housing. Rebuild instructions are in your FSM (you DO have one, right?).
If you've got the 6 1/4" booster, I have not found a good source of hard parts, but they may be out there. Otherwise two good "Go-To" names to have them rebuilt are:
https://www.powerbrakebooster.com/
Home - Harmon Classic Brakes
They also are reliable sources for many parts.