You mean the six
lower strings of a seven-string set?
Try Skull Strings. They´re designed to be detuned, and an acquaintance of mine says they´re good. I haven´t tried them, though.
However, if you´re tuning to B, I suggest you should go with gauges 13-17-26-36-46-60. I´d recommend GHS Boomers, or D´Addarios.