My driver's side sliding door won't lock in open position, so if I'm on an incline it will slam shut if not held open, I worry my little girl might get hurt by the door slamming on her.

I checked out how it works, and it's a tiny piece of hard plastic (black)that attaches to the metal, and when the door opens fully, it holds the door open, until the handle is pulled. The piece I have is broken.

Where in the world would I find a replacement one?
What would that piece be called?
Is there anyway I can fabricate a fix for it?

Any help would be greatly appreciated.