Scarce, no, but will the alternator from a '96 T-bird (with the 4.6) fit a 4.6 or 5.4 in an 2000 F150? nope. Will just about any alternator from the late 60s swap into a 1997 Chevy 1500? Yup. the problem with ford is that they have parts that were changed mid year, sometimes three or for times a year and nothing is compatible except that exact part. Time is everything in the apocolypse.
And to answer the "why would you need a car in the apocolypse?" question- to run, for defense, for basic travel, just as you need one now! If you have a truck and most people don't, then you are at an advantage. Understanding how to keep yours running is even more of an advantage.
US Petroleum reserves have enough fuel to keep commuters running for about 30 days, that's it. In addition, the military will likely be the only one with access to that petrol. You might reconsider your apocalypse transportation. If there's a gas stoppage... petrol is out almost instantly.