In typical dungeons and dragons notation, a d100 is a hundred sided die. Normally this is emulated by rolling two ten sided dice and having 0/0, or 10/10 be one hundred. It's a fairly good system for determining raw percentages.

I have however at one time, seen an actual one hundred sided die. It was this weirdly shaped plastic ball, that contained sand. It was rather strange and hard to roll. It was about the size of a half dollar, probably a little bigger if memory serves. The point was to roll it, and it would magically stop on some number. It was weird and a freakish novelty for some RPG gamer somewhere. I never bought it, since it was a little over ten dollars for a piece of cheap plastic that would eventually break.

My advice: Stick with 2d10 for getting a percentage; although you could impress all of your friends with this large and strangely amusing device.