The Sorius is only the octo (demand valve) your first stage may or may not be a Seac Sub Manufacture - if you post a pic I will be able to tell you.
Seac Sub regs were made initially by San o Sub and Salvas in Italy, (Salvas stopped machining regulators in the late 90s) and later by Seac themselves who manufactured the first stages in house based on old design models. The second stage (Sorius) is an Italian generic moulding available from San o Sub, Mega or even Salvas, hense the Sorius LP parts are still interchangeable with any San o Sub in house production demand valve.
The first stages have lately (the recent three years) undergone some changes but without seeing the unit I cant tell if its a Seac Sub first stage.
Seac Sub has removed themselves visibly from the USA market, so I do believe you may have to buy your spares from a European shop and have em post it over, or you could just get a generic alternative, Trident sells a range of generic LP seats, take yours out, have it miked for size and check with them, I am almost sure they will have one that will work.
You dont need genuine parts, any of the generic parts are acceptable.