Most of far western China is desert or mountainous. They grow grapes in the Urumqi area and Xinjiang produces a wonderful melon called Hami gua. This would not be an area regarded as a botanical hot spot of the world. Adventure Center has a 23-24 day trip along the Silk Road (goes from Beijing to Pakistan) Their no. is 800-227-8747.
Gay

Some US plant nurseries go on collecting trips to China and offer sort of a collecting tour package to China. I think you have to be prepared to "rough it" on such a trip. "Collectors Nursery" is one such business that does this. You can find them on the internet.