defrost duct

I don't think you'll find a OEM tube that isn't dry rotted. 45 years old after all.
The tube that goes to the drivers side vent is larger on one end than it is one the other. The reason they did that is so the drivers side of the glass clears a little faster than the other side ( gets more air ). You can use off the shelf tubing but you will need to buy the larger and a reducing adapter for the dash vent end of it.
There are previous threads showing where to buy the tube and adapters cheap.