Hinge pin & bushing kits are available at many auto parts stores in the Help! section. Or see Texas Mustang Parts // Waco, Texas or mustang-unl.com. Usually less than $8 for the kit with one pin & 2 bushings.
The hard part is to get the old pin out. Some were spot welded in, others were hammered so that the end mushroomed. Either way, it takes a grinder or cutter bit in a drill or Dremel tool to cut the pin or grind off the weld. Once it is off, tap the pin out with a hammer. Only remove one pin at a time so that you don't have the full weight of the door to manage. I highly recommend that you have a helper standing by to hold the door.
Once the old pin is out, lower the door and tap out the old bushings. Put the new bushings in and have the helper lift the door in place so that you can slide the new pin in. It may have to go in differently from the way it came out. That's OK, as long as you put the cotter pin in the hinge pin.
OOOPS!!!The important thing to remember is that the hinge pin isn't supposed to move once you are finished. If it does, then you will end up like me - the pin moved, it wore the door hinge instead of the replaceable bushings. Now in order to fix it right, I have to remove the whole thing again and drill out the hinge to the same size as the bushing and use 2 sets of bushings in each hinge rather than one set.
The fix for the OOOPS was very time consuming and if you aren't up to some very interesting machine work, do it right so you won't have to do it again. I fixed the OOOPS but I had $43 worth of drill bit and specialized reamer plus pulling the fender off to fix it.
my bushings need to be replaced as well, i decided to check if the door hinges i had lying around off of a 94 gt were the same as my 89 and noticed that the door hinges on my 89 were welded on the door not bolted, are all 79-93 foxes like this