Here are some tutorials on making one from mirrored reflective sheets:
I thought I'd share some information with you guys about my experiences in making ODST visors. I wanted to include this information on ZakuAce's thread but he has PMs turned off and there was no email I could find to ask him if it was cool. And it looks like he hasn't been around since...
www.405th.com
Dude that's awesome. Be super careful with your dremel lines and you've got it made! It was definitely a bit scary but worked out pretty well once I figured out how to hold it. After that and some sanding I added my primer coat! Looks pretty good, might add a little spot putty in some tiny...
www.405th.com
Your other options are going to be making your own vacuum form set up or resin printing it, but if you do not have the budget or hardware for those options, the reflective film method served plenty of troopers just fine in the 2010s before the rise of hobby 3D printing.