Where to get missing interior piece ?

[oldguy64malibu]

I have a 64 Malibu convertible that is missing a plastic piece that is located at the upper most panel, passenger side, it closes the gap between the top, aft edge of the door and the body panel.
Anybody know where I can find this piece ?
Awhile ago I asked about installing an LS engine in the car and somebody came back and said that the stock 283 was a really good engine. Well, that person was right, it is a really good choice for what I need. Ice cream and beach cruising. The powerglide is a bummer but, it works good.
What product will work to clean the instrument panel metal, aluminum ?, the kick panels also need some kind of cleaner.
Thanks for your help.