How to get a 6 month multi entry business visa to China?

Hello, I have been in China under F visa 2 entry . I am looking to get a 6 month multi entry visa. Last time I had my chinese client send me by email an invitation letter from his company , but the visa office said I needed to obtain something from the Chinese governament, and they just made F visa 2 entries. Are there any agencies that does that , for a fee? How does it work ? Thanks