I know a lot of Mustang parts places sell them. I picked mine up at a swapmeet (a guy had a kiosk full of various shop manuals). I'm gonna be honest, the 66 Mustang Shop Manual that I have is not very helpful. It assumes that I know how to take everything apart before I fix it (unlike a Haynes Manual which will walk you through taking the window out of the door to replace the regulator). Unfortunately, Haynes doesn't make a good complete manual for mustangs although they do have one for the v8 engines (all early years).
I know there are several mini-manuals that are specific to "the electrical system" or "the exterior body panels". I've never had the opportunity to use these tho I always see them being sold at swapmeets and in some catalogs.
It depends on how much you are willing to pay. Most vendors carry shop manuals. You could advertise in the wanted section of the VMF, or you could puruse the used book sellers on amazon.com, or even check out e(b)ay.