your best bet would probably be to just purchase a regular 11-52 six string set and then buy a seperate low b. You could figure out approximately what guage you need to make it balanced by using one the many string tension calculators floating around. I would figure it out for you but most of them use excel and the computer I'm posting from has vista installed meaning no excel/ word/ powerpoint.
"At least Deathcore was heavy as shit when done right, this new stuff sounds like if meshuggah and taking back sunday had a homosexual he/she creature when bred together." -- Some dude on youtube