ive spent countless hours over the last couple years trying to hunt down a brake booster and master cylinder for my 72 duster. HAHAHA yeah right...good luck on that one. not advanceauto , not autozone , not napa, not o'rieleys or any other local places had one in stock or available to order. even tried online and they are unavailable. and they may say special order or whatever. when you go to complete the checkout I found that right before your payment processes it then throws up a new screen that says unavailable ( I tried buying them a few times when i got excited after seeing they were "available for special order" lol...so I looked into it and it appeared the A1 cardone (the main remanufacturer) discontinued them. I have found them a few other places new online but they are quite a bit more expensive than what they would be from the parts store for a reman plus the deduction of a core.
however I was able to find a new master cylinder at my local advance auto....just not the booster.
you can also try looking up different years and models that would be similar ( 70-76 dusters,demons,darts,dart sports,scamps,valiant ect....ect... share the same or similar equipment over the years)....hope that helps