A lot of people use motorcycle visors because you can get them in a lot of different colors and you don't have to worry about coating them. Depending on the shape and size you need, it might be difficult to find one that fits though. You can also vacuum form one out of PETG, but that requires more tools, but it also results in a more accurate visor. If the shape is simple but you can't find a visor the right size, you can also just bend one from PETG with a Heat gun. You could also use plexiglass, but that can shatter relatively which is not very safe. To make the visor mirrored, you can either use some window tint or, if you have an airbrush, you could use a paint called alclad II, but I dont know how well you can see out of that. In theory you could also sputter or vapor deposit some metal onto the visor, but that requires a quite complicated setup to do. You could also look on etsy for a visor that would fit your helmet or some flexible mirrored visor material.