entry visa to china !

I applied for a tourist visit L at the china consulate in yemen , I submitted the required form , a hotel reservation , medical insurance & a return air ticket . I was asked to get an invitation letter from a travel agency in china . I asked to be given a list of authorized travel agents in china who can issue such invitation letter .
The answer was , they do not know , & I have to find myself .
I checked the web site of the ministry of foreign affairs of the peoples republic of china , but there is no such requirement , still the consulate insists on having this letter of invitation to grant me the L. At the same time they do not provide any help or information for locating such a travel agency.
appreciate getting any help or advice .
Thanks