Since Berea kits are the only ones that use a "B" mandrel I would start there. Also, their resellers sell "B" mandrels. Personally, I think most all mandrels are about equal in quality. Berea Resellers: exoticblanks.com , Pen Making Kits | Pen Turning Supplies | Wooden Pen Blanks , and Bear Tooth Woods . There may be others. I hold my mandrels with a collet chuck so I only purchase the rods.
Do a good turn daily!
Don