Looking For More Mustang Parts

I'm in the middle of redoing my interior also. I'm pretty sure you can't find those pieces anywhere. I'm almost certain no one makes the complete armrest, just the pad.

I'm not sure what you mean about the vinyl pieces on either side of the black seats that house the rear speakers. That sounds like the rear sail panels but those are plastic, not vinyl.

Regardless, I think you'll have to go to the junkyard.

I hate to keep being the barer of bad news on your threads.lol
I've owned Fox verts since 88.

Unfortunately just like the vertical trunk panels, the rear quarter panels on each side of the vert backseat and the armrest assembly's are not available new. Only the armrest pads are available but the others are salvage yard parts.
You can redo your rear quarters by removing the red vinyl and recovering it with your new color vinyl. You can buy and replace the lower carpet area with new carpet. Then dye the armrest pads the new color.
Beware, the plastic shell is very fragile once the vinyl is removed.
I've done this twice in the past on a couple of my verts.