Where can I buy black walnuts in Brooklyn?
I'm not 100% sure, but Sahadi's is worth a try. They have every other kind of nut, so I suspect they'd have those too.
BTW, what is with the Nutbox? They seem to carry much of the same stuff you find at Sahadi's but at higher prices. Other than the gelato (which is just ok), what's the point of that place?