Welcome to the forum.
You are right in that King Arthur doesn't make it easy to find their Sir Lancelot high-gluten flour. I went to their website the other day to reorder the flour and when I couldn't locate it quickly, I entered "Sir Lancelot" in the search box and got it that way. The other flour you mentioned, the white whole wheat flour, is not the one you are looking for. It is an entirely different flour.
The Sir Lancelot flour is a high-protein (14.2%), high-gluten flour that is especially popular for NY style pizza doughs. It has just about the highest protein content of any high-gluten flour and the specs are among the tightest in the industry (a technical guy at the company that mills the flour for King Arthur confirmed this for me). As with almost all KA flours, the KASL is of very high quality and difficult to match elsewhere. The drawback is that it is expensive to ship and, unfortunately, it is extremely rare to find at the retail level (I say extremely rare because fellow member Addicted was able to find it without having to order from King Arthur).