Belgian Linen

I see most artists use Belgian Linen for their Portraits. I've been using commercial canvas purchased at the art store. They are all made from Cotton Duck, and frankly, they seem to work just fine. I looked for Belgian Linen at some of the local art stores, but didn't find any. So can someone please educate me on Belgian Linen and where it is purchased? Thanks! Oh, I am pt pro for 2 years