I'd look at the Formatt-Hitech Firecrest ND line. Singh-Ray and Lee are good as well.
What lenses do you use? As per accessories for attaching, you'll be attached the lens and not the H6D body/back, so the accessories you will need (and size of filters) will depend on what lenses you plan to use. You could use threaded filters that screw directly on to the front of the lens, or square/rectangular filters, which will require a filter holder and an adapter ring to attach the filter holder to the lens.