Where can I find under the dash vents?

The earlier vent assembly was lots of chrome plated pot metal. Cruddy is common.
If you can buy a later model vent assembly it'll be mostly black plastic.
Then the duct of it isn't the same and wont fit the opening of your case. So...
Buy the later model anyway. Take it apart and use those vents on your duct. Yes that much of them is interchangable. To cover the black plastic with a chrome look paint wont work though. The black plastic has a vynil like surface. :(
You could to spend a small fortune on having the potmetal vents rechromed.
I choose to do the best I can with what Ma Mopar gave me to work with.
Good luck