Multiple entry visa to China - HELP!

I am an American passport holder currently traveling in China on a 90 day/double entry visa. My entry into China used up one entry and then my travel to Hong Kong and re-entry into China used up the other entry. I'd like to go to Thailand for 10 days and then return to Shanghai for 2 days before I catch my flight home. Is this possible? Do I need a new visa? Do I get it in Thailand or can I get it in Shanghai? What is the cost? Many thanks.