Does anyone know where I can find the manufacturers of the silicon valves used in the caps for p.p bottles?

Answer 1

That would be difficult b'coz u didn't mention suppliers based in which countries. However you can visit the sites given below (including an article) that provides a list of silicon valve manufacturers:
http://www.packagingdigest.com/articles/200602/22.php

http://www.pdsuppliersearch.com/results.asp?search-location=us&search-type=prodcat&search-text=silicon+valve&stype=Products&searchtext=silicon+valve

http://www.thomasnet.com/

Answer 2

Standard Microsystems Corporations in are located in Hauppauge Long Island.