I plan to travel to Beijing, Xian, Luoyang, Guilin, Shanghai and Hong Kong. Where can I find the best value on private tours?

best value on private tours that don't include hotels (since I have specific hotels I want to stay at)? I have read it is cheaper to arrange city by city, as you go, so you can deal directly with the local agency that will provide you the service. How can I contact these licensed agencies? Any recommendations?
Thanks!