Try your local Ford dealer for your vacuum booster, for I had to put a booster on my 78 F-150 Ranger Lariat. My recommendation is while you're at it is to put a new master cylinder while you're at it, for that is what I did on my 78 and it took care on my braking issues. That's just my thoughts, but it's well worth it to do both at the same time. Hopefully you find this helpful to you. Also talk to the service manager about it and listen to what they have to say about it.
That's the same way mine was until I had the work done on mine, it had felt after the first pump or two the pedal felt real hard, and felt like I was trying to stop an aircraft carrier, or a freight train. So in your case just replace the vacuum booster, for you've already done your master cylinder .
Mac's Antique Auto also offers a booster rebuild. You send yours in, they rebuild it, and send back. I seem to remember a price of around $200-ish. This is a great service if you have a hard-to-find booster.