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.