Western Digital 2.5" 500GB Portable Hard Drive for $65.00 shipped.  Here is a screenshot for your reference.  Remember to select free upgrade to 2nd business-day delivery.  This 2.5" portable drive does not require a separate power supply like most external drives.  It is also smaller than most of them, making it more convenient to carry around.  If asked during checkout, simply use your name as the business name.  Most places have it for about $99.