I have walked intensely in Milano during three days one month ago and I haven't noticed the least bit of a shop selling films. If I had, I would remember it. For instance, when in Prague, I didn't miss Foto Skoda or Foto Pazdera.
My advice would be that you take a large stock of films with you before you go to Milano.
Milano is famous for the reputable "Fotocamera" collectibles shop, but they don't sell films...