If you know someone who lives and works in China I would definitely suggest trying to find local companies to help, these are going to have the best rates and be able to help you within your niche.
If you don’t then you are stuck with looking from the outside in, which can be a tough enough job when you have a language and cultural barrier to contend with, along with the fact that China has its own equivalents for things like Google and Facebook.
For general purpose advertising these are the three sites that kept coming up in my research, they are all big companies and generic but unfortunately they are the only ones who seem to care about selling to non-Chinese advertisers.
http://www.adsage.com/agency - Probably the biggest Ad Agency in China, have partnerships with Baidu
http://www.tencent.com/en-us/ps/adservice.shtml - Owner of a huge Chinese portal
http://ad.sohu.com/ - Another huge portal in China
Feel free to comment with any you have come across!