Boot disc is most easy if you don't mind swapping, but since you say you don't wanna just search google for dreamcast mod-chip or dreamcast 4 wire mod chip and you'll find some.
it's nice they are sold for pretty cheap. usually about $15, but once i saw it for as cheap as $4! I wish i remember that place. I would mod my dreamcast but you have to solder 4 wires, some of which is of the pins on the black chip things (whats the real name for those? and if anyine has the time, what do they do?) and i'm afraid to solder to those caus ei ruined my saturn that way.