If you have already return to Turkey, then you are unnecessary to find an visa agency. You can submit following documents to Chinese embassy in person.
1. A passport valid for more than six months before expiration and at least one blank page in it. and Copy of information pages of the passport
2. One complete application form (The People's Republic of China Visa Application Form) with one recent passport-size photograph (black and white or color) glued thereto.
3. The invitation letter from the inviting unit or the visa notification letter from the Authorized Unit are required, together with an recommendation letter from the applicant's own company or institution signed by its general manager or owner, the true copies of the registration and taxation certificates of the company (if), and the certificate from the local Commercial Chamber.