Hi Matthew, I live in a small fishing village on the coast of NC , and can find the oil everywhere. NAPA, K Mart, Walmart, & Advance Auto Parts, just to mention a few. Several of the brands call it HD [heavy duty]. Good luck in finding it. When I had a motorcycle shop in the 70s, 40wt was used in a lot of bikes, and I would think the Harley dealer would also stock it.
Have fun,
Quenton