Hi
I know about these visa issues because I took a double-entry tourism visa to come here and I've just been to Hongkong last week to renew it!
I think you can also go to Hongkong to extend your Visa.
I have some friends who came to travel in China, they land in Hongkong and spend few days there, without any visa and they go to this place to do their visa for China:
CHINA RESSOURCES BUILDING, 26 HARBOUR ROAD, WANCHAI, HONG KONG
They wait for 4 days to get it and payed around 200$HK for their Visa, which is really cheap!
If you need more informations, feel free to ask me!