I did a google search and found the Thorogood website.  On the right side, there is grey lettering "More information and checkout". Click on that for each style, and you will see whether that style has a 12B size.  Not all boots on this page has 12B, so you will have to look.

I also found a website for Shepler's.  They have Ariat pull-on boots. 

I don't know anything about either of these brands, other than I found them thru a Google search.

Workboots website brings up the following.  They don't show a B width.  However, they show N and XN, which I presume it Narrow and Extra Narrow.  Again, that is presumptive on my part. It appears that most brands on this website is Justin or Thorogood, with a couple of others in the mix as well.