i think sometimes they make sets out of different gauge strings from 6 string setsand stuff like that, im not positive but ive heard this
For 7 string (B-F tuning), you can order the Will Lee signiture set from Dean Markley. Other than that, you have to order individuals.
Where does one get a set with a low F# (string below the low B)? What gauge would a low F# need to be?
You don't really get sets with a low F#, you have to order them individually. List price for a Rotosound .150 is something like $12! The gauges vary, anywhere from .150 to .180. John uses a .165. I don't use a low F#.