Hi Hamid,
If I remember correctly there were two different types of sun visors. Mine has a plastic, shaped like a little buckle. I think yours have about a 3mm bar across the gap and functions like a small hinge which clips into a plastic part screwed onto the top of the windscreen surround.
You could start with a similar sized rod or bar, about an inch or so long and see if you could slide it into the bracket situated in the gap. The bracket is not clearly visible. It is part of the wire frame of the sun visor and slightly hidden by the upholstery.
Googling around the internet, might give a an idea of the part you are looking for.
Good luck.
Chris