Look here for Vented Loops and Diverter Valves specifically designed for marine head plumbing and waste management use. Built of chemical and corrosion-resistant materials, these parts are constructed for significant odor control as well.
Other Diverter Valves or "Y" Valves for waste management use can also be found in the Plumbing Fittings Category of this Plumbing Section - use this link: Valves - Marelon & Plastic