I think fabrics n stuff sell buckles, definatley sell webbing and you guessed it fabrics. Webfittings.co.uk sell buckles and webbing. Have no idea what other colours these companies do.
I was looking for quick release buckles which are adjustable on both the female and male part so I don't have to permenatly stitch one side to an item. The only place I've found is pennieoutdoor but at 81p ea. [I think] could be expensive if I want to get 20 or so...