[Arran]

The silver micas are probably viewed as a specialty item over at Mouser, lower demand hence a higher price, most electronics manufacturers have been switching over to NPO rated capacitors of various types, and have been doing so since the late 1930s. Those components that you run into in sometimes that look like dogbone resistors, but are hollow, those are early NPO rated capacitors. Most 1930s and 40s radios didn't use silver micas, they were just plain old mica capacitors, a sandwich of copper or brass and mica sheets, which is probably why they are so robust, even though they can still fail.
Regards
Arran