How do I repair metal sheath on top of the threshold of sliding glass door?

I have a very well constructed, large, metal sliding glass door that leads to our patio. It was installed in the 1960's. It began dragging a bit. Upon closer inspection I noticed that there is a very thin sheath or sheet of metal that is in full contact and overlays the threshold. We pulled this sheath away last year because it had a jagged edge. Now the door won't slide and it doesn't "roll" anymore. It is as if it went from 0 co-efficient of friction to trying to push a car with no wheels.

WHERE DOES ONE BUY THE PIECE THAT OVERLAYS THE THRESHOLD? HOW DO I ADDRESS THE ISSUE OF THE DOOR NOT ROLLING EASILY? MY SENSE IS THAT THE DOOR HAS MIGRATED DISTALLY. THE FRAME JUST SITS ON THE BARE METAL AND DOES NOT SLIDE ON IT. help!! YOU CAN BARELY SLIDE IT. I AM BIG AND STRONG AS WELL AND STILL FIND IT A CHALLENGE.