ISO pumpernickel flour in North shore
There is really no such thing as "Pumpernickel Flour" that is just King Arthur marketing lingo. If you look on their web site, the ingredients on this are just "Rye Flour."
Arrowhead Mills, and I think Bob's Red Mill both make excellent whole grain Rye Flour that you can find at either Market Basket, or in a pinch, whole foods.
I have seen King Arthur pumpernickel flour at Whole Foods, but I'd call first to check. Some stores carry Bob's Red Mill Products, and if they do, they may have pumpernickel. I've also gotten Bob's products on amazon.com.
If you just need a little bit, try a bakery that makes pumpernickels, ryes, or brown breads, and ask them to sell you a bit. But if you want to have over a pound on hand, I'd try the aforementioned methods. Good luck!