For anyone else who's looking for these -
I believe that Prisma discontinued their set; I got mine at a Staples couple of years ago; I still saw them there a month or so ago, but I think they just had some old stock. I know the numbers that match to the metallic colors aren't listed on Prismacolor's website anymore.
Dick Blick has a set by Derwent
(HERE) that I also have and I can't tell the difference between coloring with these and coloring with the Prismas. I am NOT, however, an artist by any definition of the word. Someone who knows better might think differently.
I also have a set that I got at Hobby Lobby that's only 5 or 6 pencils. I don't remember the brand, but they are bigger around than a regular pencil; those work pretty well too.