I'm presuming you want an "already done" unit for quick installation?
The BEST option is to send your booster to what used to be "Booster Dewey", for rebuild. Dewey has retired and a former employee is carrying on with great product rebuilds. In a different town, too. Google can find the contact info, but it might also be found in the various threads in here on "power brake booster rebuilds", too.
The booster that came on the car was used from about 1965 to 1970 on C-bodies. Power drum brakes or power front disc brakes IS important, too.
CBODY67