Searching for that part gets a lot of hits on Google. But sizing orifices like that is a little more complicated than you'd think. Apparently, you need to consider your altitude, as well as the fuel source. It's true that you want to use a smaller orifice for LP gas than for natural gas, since it has more BTUs per volume. But if you don't figure the altitude in, bad things can happen: http://enora.nortekhvac.com/literature/7092130.pdf