Where to get heater to vent air ducts?

My 71 Duster is missing quite a few things, and one thing I notice is that its missing all the ducts that goes from the heater box to the dash vent.
Its a non AC system and I was wondering where could I get new ducts or any information regarding this?
I didn't took the car apart so i don't know how its routed through the dash.

A diagram would be nice, anyone as a link to one?