I bought my extra 3G cable at Wal-Mart, it was expensive $19.99 but its an OEM Apple part.
The sad part is when I exchanged my iPhone yesterday the Apple guy gave me the cable, power plug and earphones out of the new box even though I never brought the old ones in. Too bad I don't still have the box for the one I bought at Wal-Mart. Now I have 3 sync cables...