There is a boot disc. It's not very useful unless you have an original. That is, an original of the boot disc, not any original game. The disc only disables the copy protection after it's been loaded normally, so a copy of it can't bypass the copy protection. Sega distributed them to third-party developers so that they could test their games in standard Saturn systems.
A friend of mine has a boot disc, which he won't let me have - he's keeping it for "nostalgia". He was responsible for two of the poorest games on the Saturn - Revolution X and Doom. To be fair, Rev X was his first go at programming the Saturn and was also a pretty poor arcade game, and Doom was done in less than 6 weeks from scratch after another developer had spent months trying to do it.