Bump stops are usually reused stockers or aftermarket parts specific to the application.

On some cars, the bump stops are critical to the handling of the car. Miatas, for example. Not sure if the Mustang suspension is that "sophisticated". But, if it is, then the bump stops are designed to be progressive and act to increase the spring rate predictably at the limit of body roll. They can affect the over/understeer balance of the car in hard cornering. If you're running stock springs, this could be very relevant.

You'll probably have to go to Koni for the metal bushings you're talking about. That, or find a friend with a lathe.