Get it here → http://www.stepmania.com/wiki/Downloads
My god this game is so addicting, and it’s free unlike most other versions of crack, so you don’t have the excuse of not having enough money for it. I only downloaded it today, but I’m already addicted. I would recommend getting a theme for it that replaces the arrows with bars, it’s much less frustrating that way.
There are a large number of song packs and mods, make sure you look through those.