Ive had good service from
Harringtons , McCullogh& Wallis and and ther was another in Nottingham that I've forgotten the name of, something like Bridal Lace & Fabrics. I did used to get silk from Hong Kong but the supplier sadly retired and they were the best!
Re the dress, Organza is not really floaty, although it is sheer; it isn't overly stiff, but definitely not what I would call floaty. If it is floaty you are wanting, I would suggest chiffon. It's a beautiful style dress.