South African, in USA on 8month internship. Didn't realise I could only stay 6 mnths at a time,now 2 wks over that. What 2 do?!

Asked in Sophia, NC - 10 months

I applied for a US visa in December 2011, was granted a 10 year multiple entry visa. At my application interview, I informed them that I was going to be in States for an 8 month internship, so when I was granted the visa I assumed that it would be alright for me to stay the full 8 months. I had a strange feeling about it last wk though, & after some online search I discovered that I am only allowed to stay for 6 months at a time (unless I'd applied for an extension of stay) - I wish I had been properly informed of that earlier. The stamp in my passport (with date of entry) has a blank for date I should exit by,has not been filled in. Who do I contact? Immigration? Customs and Border Protection? Return flight is 22 Sept, when internship ends. Can't afford to purchase another flight. Advice?