where can you get the panel that covers the jack behind the seat

If I understand your question correclty, it's called a curtain, made out of carpet, and you can buy them new at pretty much any outfit that sells carpet sets for Elky's. Might even find one on Ebay.

Here's an example. Look under Accessories about half way down the page.

http://www.automotiveinteriors.com/carpetcars/chevy/59_87elcamacc.htm