Hi,
For a film project we need to have some sort of wizard with a grey beard (for example like Gandalf).
But if I look on the internet, I always find those fake & cheap looking carnival things.
Does anyone of you know a site that is selling good looking movie props or stuff like that ?

What you are looking for are film & TV make up suppliers. These sell the materials for professional make up artists, having said that, you may have to learn some techniques in applying them.

Here's a UK supplier I googled: http://www.backstageshop.co.uk/acatalog/Make_Up.html

I expect a search may find one closer to you.