The main ingredient here is retroreflective black 3M adhesive vinyl. In other words, a big black sticker that shines back at you when you shine a light at it. It's awesome stuff, great for putting on your bike when you want to be a safer rider at night without looking like Safety Boy during the day.
Small pieces can be easily found on eBay. I got a 6" x 18" piece for about $10, including shipping. That's enough for 4-6 bumper stickers depending on how tall you want them.
eBay search for reflective tape