Why is it really hard to find parts for a 1984 mustang?
Shouldn't be that hard to find parts for. The reason you see so many of the other "fox body" style mustangs (79-93) is because it's a highly popular car, and most people choose the later style (87-93). Nonetheless, and no offence intended, the 84 has got to be one of the ugliest ones of them all.
Otherwise, most factory replacement parts should be available at your local auto parts store, if not, then your store sucks and you should shop elsewhere. I own a 79 and have no problems finding parts.