Im not sure what they cost because my work had some laying around that they had no use for. So it was a free mod for me lol. But I'm sure you can get em on ebay. Or try googling 'lighting colored gels' or something.

I also used 2 layers on my old Sport Trac overlays when I modded it blue. It works really good for that EXCEPT that you'll get major hotspots if you don't use something like 360 degree SMD LED bulbs.