A breakout board is a PCB interface between the parallel port connection and your driver boards. Here's an example :
http://www.embeddedtronics.com/breakout.html
They have an opto isolated version too.
Why don't you simply add a second parallel port to your computer? You can have up to three of them. I am using a breakout board from embeddetronics and two of their MicrostepPLD boards. That's good enough for 4 motors. By doubling this up I can easily go to 8 motors. The only thing I need to do is get me a cheap parallel port card. You can generally pick those up second hand for a penny on computer fairs.
Heck, I could even go to 12 stepper motors if I needed to. Not that I'll ever find any use for that (up to now).