Your only source is from a donor car as I've not seen many aftermarket 73 Mustang sheet metal parts. To further complicate your quest, I don't know if that part is the same as a 71-72 Mustang or if it was common to any other Ford or Mercury (my parts book covers 65 to 72 and it is not serviced, 65 to 70 are, strange). Go to a Ford dealer and see if they might have a Ford Car Parts manual for 1975 on. The base Ford number should be 3486.
Jim