You mean the kind that goes on windows? Ask for a scrap at a place that tints car windows or house windows, or buy some at Home Depot (it comes in very large rolls, though).
they look nice, gold and silver, but after having dug around a bit it turns out that, being made for safety after all, they are actually lined with thin layers of gold or silver etc... I think the silver and gold often run very expensive.
Please if anyone finds a cheap one, I would love to know
That visor in the link that I posted runs about $40, I believe... It's a lot more than the standard $6.00 visor, but it's definitely a unique item, and cheap in comparison to the prices of most custom helmets owned by members here.