BCinRI, you want a Watts valve for a hot water heater, not a pressure relief valve for a boiler. They look very similar, but are different items.
You can buy a watts valve at a good hardware store or plumbing store. A watts valve will relieve with either heat, or pressure or both.
The 30# PRV for a boiler is just a pressure valve. A boiler has a pressure reducing valve manitaining 6-8psi to the water in the boiler. ONce the water is heated, it expands and the pressure rises. There is an expansion tank to provide room for the water to expand. The pressure rises to around 12-18psi, depending on the volume or water, and the capacity of the expansion tank.
My Propane boiler stays at about 10-15psi when hot.
So find a watts valve, and don't buy the boiler relief valve, it isn't what you need.