Do you want Nickel Plated Steel or Stainless Steel?
The Ken Smith build your own set option will give you the option to pick Standard strings or Taper Strings for what is all available. The Build your own Set options are basically all the singles available put into one place so you do not need to click 20 different pages...
Also, since you only want a Tapered B, you are better off buying a standard 4 string set and adding low B's to that.... (cheaper!)... If orders are placed for say .045 .065 .085 .105 .125T in a Build your own Set option, I credit the difference saved and ship the .045-.105 set and a low B...
Same thing works if you want .030 .045 .065 .085 .105 for example...
OP: It will be better for you to get full 4 string sets and add low B's... There are Tapered Low B's available from GHS, Ken Smith, D'Addario, Fodera, and DR.
If there is a single string you are interested in and do not see on the website (yet), let me know and I can get you taken care of.