You can submit your HK visa application to Chinese embassy there. the duration for a Tourist visa is 30 days in generl. the documents are as follows:
1. The original passport with a validity more than six months and enough blank visa pages;
2. One copy of completed visa application form in Chinese or English (two copies for US citizens), attached with one of the applicant's recent colorful front pictures of upper body (30mm X 40mm)
3. If necessary, application for travel visa needs the certification of the applicant's identity, round-trip (vehicle, ship) ticket or travel check or bank saving statement(enough for the trip: about USD100 per day).
4. Your valid Thailand visa.