74 charger inner support for rear quarter

[jyancey74]

Need help finding the inner support for the rear quarter on a 74 charger.  Mine is rusted out but I don't see anything online.  I am installing new quarter panels (or at least pieces of the new panels) but my passenger side is rusted out through the rear quarter behind the rear wheel all the way through the inner support.  Drivers side is good but need to replace passenger side. Can anyone tell me where I can find the inner support for the passenger side quarter on a 73-74 charger.