NOS A/C was darn near impossible to find for them 15-20 years ago, when I was looking, so I can imagine you're in for a hunt. I recommend setting up an Ebay alert for "1964 f100 air conditioning" - it'll send you an email within an hour or so of one going up for sale.

They have a few parts on there now, though I don't see a complete unit. Some parts are shared with cars of the era.

One thing to consider, once you go with any options the truck didn't come with, whether the AC is factory or aftermarket isn't going to make a difference on it's value. Only if it was a truck that came with factory air. So you might want to check into an aftermarket unit from Vintage Air.