A tail panel for a ‘75 Dart Sport? I don’t believe they’re reproduced at all, so you’d have to find someone that’s parted one out. Or change to one of the panels that is reproduced, but then you’d have to find tail light housings and lenses.
You might try asking @Lee Robinson, although I don’t think he’s listed any tail panels recently.
Depending on where your panel is “rough” you might also be able to buy one of the earlier tail panels and use it for repairs, or swap the later tail light openings into the replacement panel. But that could be a lot of work depending on the condition of your tail light panel and how much sectioning you ended up doing.