The "BC 1.37" is the threading designation and is the very common English threading. The "68" is the bottom bracket shell width in mm and is the standard for English threading too. I presume the "CB" stands for cartridge bottombracket. Your crank has a JIS taper but these are very common also.
So any English threaded square taper bottom bracket of the proper spindle length is what you need. IIRC, the Ultegra 6500 double crank required a 115 mm spindle length but a 118 mm will also work without compromising the chainline too much. Check with any LBS or Loose Screws has Shimano UN 54 bottom brackets for only $20.