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Mil1ion is right, a donor would be the cheapest. However you need to look around at some junk yards, you might get lucky.
I sold all the parts needed to convert to disc brakes on ebay back in Feb. for 150.00 plus shipping to a guy doing the same thing you plan on doing, the set I sold only had 48000 actual miles on them. Calipers, rotors, king pins,spindles, hoses and master cylinder. I converted my truck to an IFS setup and no longer had use for the parts.
Keep looking you will find a set and good luck with it.
4x4 or 2wd? I have a 1973 4x4 and it has drums in front. I think you just have to get knuckles, spindles, caliper mounts, calipers, lines, rotors, hubs and proportioning valve from a '76-79 f100/f150- at least that's what I'm planning on doing with mine.