Olson’s Gaskets has virtually every gasket you can imagine for early American cars, including the Ambassador engine.
I purchased valve cover, side cover, and fiber exhaust manifold gaskets from them. Not cheap, but everything works well, and they stand behind their product. There are two different gasket sizes for the valve cover, so you need to be specific. I got the wrong one on the first shot, and they sent out replacements as well as a couple of spares at no additional charge.
good folks to work with: http://www.olsonsgaskets.com
Port Orchard Washington (360) 871-1207