Well I was just cuting out a rusted section of my floor board (1985 bmw 318i) to get it ready for new sheet metal when my saw goes straight into the small brake line that runs along the firewall. Needless to say I need a new one. Any help in finding one would be much appreciated.

Most automotive parts shops sell pre-formed straight brake lines, in a variety of lengths. You can easily bend them to shape, or approximate shape. Be careful not to kink it. It is fine to have a larger radius curve, and approximate the run of the original line. Be careful that it does not rub or vibrate on something it should not.

Good luck