You need to check out Fuisck and Kanter. They have all sorts of parts for your vehicle. I am currently going through the air plenum in my 64 98 and it is a PITA! If you check out the following websites you could buy a set of shop manuals that should have a vacuum diagram and description of the way your air doors work to plumb heat to the defrost or cold air to the vents if you have AC. Don't forget, an AC air plenum and a non AC air plenum are two different animals. If your car didn't come with AC then you will have to find an AC car to salvage the air plenum and blower motor out of. Give more details and we will help. If you have AC and it just doesn't blow cold chances are you just lost your refrigerant. You could always do a recharge but it won't cool as well with R134a.
http://www.fusick.com/
http://www.kanter.com/