ESAB and Lincoln both offer a variety of flux wire combinations that will work very well for a 5L B material. What to use in the final analysis will depend on the properties desired in the weld metal, workmanship issues, and relative cost. The chosen wire and flux combination should work together ( eg: don't use a manganese and silicon active flux with a relative high Mn and Si wire ) and it is not advisable to mix manufacturers at this level of material performance. Depending on your material property, inspection, and dimensional acceptance requirements, the cost of workmanship/quality preformance can easilly out weight the cost difference between competing consumable choices. 5L B matieral is typically not esoteric ( "low carbon steel", but make sure to check that C level) , and a call to the sales rep will quickly give you a short list of flux/wire combinations appropriate for your application.

For the benefit of others who may answer, you need to give comprehensive info about your particular application.

And, if such is still timely information, and if in the US, call Labarge (US distributor; do an internet look up) and see if they have what you need for less than what it would cost to make.