Pretty much any welding shop that carries Miller can order that for you, or you can go straight to Miller to order it. Go here and enter your serial number:
Just called Miller and that part has been discontinued since 2011....so I know this is a long shot but does anyone have one or possible know where I could find one??
Also, it might be worth talking to the most experienced counter guy at you local welding supply house that sells Miller, he might know something that others don't
The guy I talked to said he called Miller and it was an Airgas branch, I just find it hard to believe that that part would be no longer produced since it was on so many models.