Here is an Ebay auction for a used secondary carburetor booster by Ebay member "magmustangs" with a Buy-It-Now of $199.00:
1963-1967 Ford Mustang, Shelby GT350, Fairlane, Mercury Comet. Original 289 HIPO Autolite 4V Carburetor SUPPORT - Secondary BOOSTER VENTURI. This is an ORIGINAL FORD PART. Has BA marking. Looks Good. No guarantees on working condition.