Where can I find photgraphic plates and films for spectrographic analysis

Try exploring Kodak's Web site:
 http://www.kodak.com/
It makes a wide variety of scientific emulsions.

You're unlikely to find glass plates. The demand for plates is so low
that Kodak is now making them only on request, and they are extremely
expensive, based on my experience with a professional black and white
emulsion for astrophotography a few years ago. It does make many
scientific films in sheets, however.