ANSII uses Linux. Linux is open-source software. This means that not only Linux, but also the entire source is available free of charge, and you can modify it so that it suites your need. Although most of the people won't need to change the source of Linux. Many companies bring their own distribution of Linux which you can download for free on the internet. Those distributions usually work fine for most of the people.
You can also buy a copy of some Linux-distributions at ANSII on CD-ROM for € 2,50.