Hello
Please where possibly can I get this plastic locker piece at the edge of the sunroof lining?

Mine has expired and been broken apart, needs replacement.

 

that piece is a vinyl strip sewn to the front of the headlining fabric used to pull and "tighten" the fabric to the interior side of the sunroof panel.
Once the old headlining fabric is replaced, it is difficult to replace it.

What I did in my car was to use contact cement to fix the front edge of the fabric pulling it and folding it over the front top edge of the moving metal panel, clamping it until dry, then I screwed the vinyl strip over the exterior metal panel and glued fabric with the 4 screws.

these vinyl strips are used in car seat upholstery to tighten the seat covers over a metal "lip".
A bench seat in a Van will have the needed length of vinyl strip.