Hi zexpress,
If you look under the polyfoam section of the component thread here, you’ll see some good options of which I am aware for polyfoam. I went to a few of their sites and it allowed me to enter thicknesses of .5", but I would advise a phone call to any company you might be considering, as you are requesting a very thin piece of foam.
If you are considering ordering from Foambymail (AKA FBM or Foam Factory and other names as well) then I would read this post and this post and this topic (about their polyfoam and sources) and this post (presumably from a past employee) before buying anything or considering them as a reliable supplier that provides accurate information about their foam products.
Phoenix