I am interested in finding Du Huo tea that is claimed to be good for arthritis.

Du Huo Tea actually is considered as a kind of Chinese traditional herbal medicine. As you siad, it is good for arthritis. You can find it at some Chinese traditional medicine stores, such as Tongrentang or Zaolutang.
As for the other kind, do you mean the Huang / Yellow one? If so, it can be bought at any good tea store of China, especially in the tea stores of Anhui and Sichuan.