I picked up a USB cable for my 6750 so that I could sync at home and work. Problem is when I try to connect with the desktop manager, it doesn't see the Blackberry. I picked up the cable on eBay, it didn't come with any drivers (don't know if any are needed)
There are a handful of USB to serial cable adapters that RIM has certified, but you still need the correct serial cable or cradle, as acnst has stated.