Making clear eyeglass lenses into sunglasses - who? where?

I have an older pair of glasses (clear glass). They are sort of my ‘spare pair’.

Some time ago I heard about a service where some company would take the lenses out, immerse the lense in some tint of some sort, which would, essentially, make sunglasses. My recollection is that the dipping service was cheap – maybe $10/lens – far less than buying a new pair of prescription sunglasses.

But I have no idea who does this sort of service. Any suggestions?