I lost the Nahkriin mask; How can I get it back?

I've completed Skyrim oblivious of the Nahkriin mask you find by killing the dragon priest of the same name, Either that or I've picked it up and dropped it somewhere or sold it. I can't really remember what I've done with it.

Is there any way at all for me to go back on my save where I did have Nahkriin and restore it somehow and go to my most recent save and have item? I have progressed really really far in Skyrim and it is a massively large space between when I was able to get the mask (or did have it) and where I am now.and if i cannot retrieve it is this something to be bothered about or should i happily continue my game ?

I'm on the PS3, so solutions involving the command console won't work. Solutions which work for the XBox might.