On April 1, 1936 a special designation was Introduced for the Jungsturm of Nurnberg which was in the tradition of the Jungsturm Adolf Hitler, a "J". An acknowledgement of the Jungsturm. This is why the Bann is designated Bann "J", instead of numbered.
Source Handbook of the Hitlerjugend By W. Saris page 458 and 459.
I see that you are a Gold Member of the Forum which allows you access to the Bann number/triangle lists. You can find bann J on this list:
Bender Publishing at San Jose (USA) will help you to get the book. Look at their website.
However mailing it from the USA is quite expensive!
As far as I know Weitze at Hamburg does not have any available.
vdmedien24 (nickel) at Zweibrücken does have it, but is rather expensive (149,- euro).
Maybe you can get it even cheaper at Bender Publishing, including their shipping costs.
Thanks. It was one of the earlier caps I purchased when I started collecting MHJ caps. Still in contact with the Australian collector who sold it to me all those years ago. It pops up on the internet for sale from time to time, this exact cap, I have no idea why. It is in my collection and not listed for sale.