Usually an invitation letter is not necessary to apply for a L-visa to enter China ("L"-Visa is issued to an alien who comes to China for sightseeing or visiting family members/friends.). A usual way to apply for a L-visa is simply to go to the Chinese Embassy or Consulate -General to ask for it (see here http://english.gov.cn/2005-08/29/content_27141.htm).

In case you really need an invitation letter, please let us know by email. We will send it to you.