Yeah, MarsAttacks were very nice, but no longer makes them. Every now and then they pop up in the PX but are often pushing $150-200 (or more?) for one in good condition. The main thing about his were they were metal. Chef makes 3D printed ones. Spat has the cheapest option, but I've never owned one of his so I can't vouch for their accuracy.
Overall, for better images, look no further than this thread. Harry posted pics of the screen used helmet liners to show how the original IR sight mechanism worked (spoiler alert: it worked with rubber bands!).
viewtopic.php?f=3&t=15585&hilit=rubber+band