That is the real question, isn't it. Basically you can make one out of anything that is a transparent plastic, or you can get clear plastic, make a mold, and vacuform the shape you want. Or you can purchase one if you want, but I'm not very experienced with that end of things. So it depends on how much time, work, or money you want to spill into it.
Either way though, we are all here to help and a lot of us have more experience that I do myself.
It may prove useful if you want to build a Spartan helmet, if you scale it SLIGHTLY larger than normal and install some fans to circulate air and keep your visor free. That way you won't feel so claustrophobic.