How to get the Invitation Letter from Chinese Tourist Bureau directly without joining Travel Agency?

We can book hotels, Air tickets and Chinese Travel agency online. But when proceeding for the visa mainland China ,the Chinese Embassy in Sri Lanka is asking for Chinese Tourist Bureau Letter. For that letter local travel agents charge huge money.We need to clarify why it is so difficult to get it independently from China. If any of Chinese Travel Agent can help us we will be thankful and also ready for the payment. Can we write directly to Chinese Tourist Bureau? If so please send us the mailing address.Thank you.