How to get an invitation letter from Chinese Tourist Bureau from Sri Lanka?

Hi- i am planning on traveling to China as a tourist, to meet some friends and sight seeing in Qingdao and Guangzhou. i have booked my tickets and hotel accommodation in both cities, but now my travel agent said i need to get an invitation letter from PRC Tourist Bureau. how do i do this?

i plan to fly from 27th of March to the 2nd of April 2016.
Thanks,
Romesh +94773905003