Why is it so hard to find

I have a 1965 Dodge Dart 225 slant 6. I just had the engine and transmission rebuilt. The radiator works fine around town on the highway by the gauge it seems like it is running hot. The engine didn't spew any antifreeze. I have a new thermostat, water pump, new hoses, etc. the radiator seems to be a hard one to replace both the inlet and outlet on passenger side. I measured it across 19" X 18" is that the stock rad? The fan is a four blade fan with a new belt. I want to get a new radiator but no one has one.....I thought maybe it wasn't the correct rad but everything lines up perfectly.

Thanks to all who read and answer...