I have made them before. and seen them elsewhere. The ones that I have seen are on a product called lexedge.
lexedge is a material that comes on a roll and is printed on via a Gerber Edge (foil press).
They can be on coloured translucents, matte or glossy, frosted finish or smooth.
I dont know how expensive they would be to run a whole lot of em. But you could sure check in at any sign shop.
Just ask if they have an Edge2 and if they do then they can do it for you.
If you wanna save cash then tell them to just give you the finnished sheet and you will pop em out yourself. they are scored by a plotter so no cutting is necessary.
I dont see why this cant be done on any press though. All they have to do is order the material.
ps. avoid light colours because they have to be backed up with white to achieve the proper value. This means hours in doing the trapping, more missed registration, and an exra colour plate (or foil) $
Dolomite, the tough black mineral that won't cop out when there's heat all about.