As things turned out, I contacted Chemplex about the Mylar film and explained what I intended to use the film for. I received a response today and it was not what I was expecting to learn. The film samples that they offer are not metallized! The person who had sent me that link had not noticed this fact and neither did I when I decided to contact Chemplex. So, today I did a search again and did find some 0.5mil thick metallized Mylar sheets on Amazon and have ordered some. eBay was no help, nor was Alibaba, the Chinese source. Alibaba sources do have some metallized Mylar film in the range I could use, but their minimum orders are astronomical as to quantity and the cost is far beyond practical for this type of endeavor.
The order is supposed to arrive May 29th. and consists of 50 sheets of 18" X 30" metalized Mylar of 0.5mil thickness which is the same as 12.5um. It is about double the thickness I wanted but appears to be all I can get for now.
Joe