We tint lenses all the time. You can actually tint an acrylic lens using synthetic dye, and boiling it for a few minutes.
On ebay you can also buy replacement lenses for ski goggles, motocross goggles, welding shields.. It all depends what size you want.
comes flat, so it will bend to whatever curve and are very easy to cut to whatever shape/size you need.
can buy replacement lenses for sunglasses, which are more expensive and you have less of a guarantee that they'll fit..[/url].
(Bearing in mind that I didn't bother to look on Ebay.com, but just on ebay.com.au.