Sent it to DELL to claim for warranty if its covered. If not then you have to leave with it.
Here is a store that sell surpluse DELL parts. It sells notebook and desktop motherboard from DELL.http://www.discountelectronics.com/
You need to be very careful when you decide to replace motherboard. USB connectors are directly soldered on the motherboard. I know in CA i can walk to electronic shops and possibly find a replacement USB connector. It defends on your location.