You can get the basic repair parts (brakes, starter, alternater, etc.) at most parts stores. Chances are you will have a local Mustang only shop in your area, I've got three in a short driving distance where I live and several more further away.
Other parts (interior & body) you can find a number of sources here:
A lot of parts are after market buy you may want to search for original ford parts for some items like sheet metal for a better fit.
I've found a lot of parts on ebay, with a little patience.
You'll want to subscribe to Mustang Monthly for tips and help.
Joining a Mustang club is a good source of technical help.