I think teaspring.com has a very excellent black tea selection. In fact.. all of their good black teas have taste close to a good Keemun. I have myself tried: Keemun Xian Zhen, Bai Lin, Sichuan Gongfu, both Yunnan Golds, Zhu Hai Jin Ming. It'd be hard for me to pick a favorite, maybe Sichuan Gongfu. All of them were a bit uneven - really great when they work but maybe I'm not very precise in my brewing, sometimes they would not work as well. I never tried gong-fu preparation with them. I mostly use two large glass pots, brewing without an infuser. These teas are inexpensive and for their quality, in my opinion, are a great bargain.
houdeasianart.com also has a pretty nice golden Yunnan, their advantage is that when I order, I get the package in ~2-3 days in NY area. It takes a week or a bit longer from teaspring, since they ship from China.