What are you planning to do ? there is little or nothing in the UKregarding Parkerising/ Phosphate products, I tend to mix my own, ( and it always varies) the basics are Phosphoric Acid, Manganese dioxide and wire wool......... this gives a grey to black finish.
You can use Zinc oxide instead of the Manganese, this will give a silver to grey finish, never used Zinc as yet.
There is a cold Black oxide kit that is very good that I use on small stuff such as bayonets etc, gives a black finish. see link bellow
Caswell Europe: Online Metal Finishing, Restoration & Renovation products