Where to find an iPod's serial number if you don't have the iPod?

You can find it on the box and the sales receipt if you still have them with you.

 

Itunes can tell you the serial of the last iPod connected.

In iTunes 7.6 or later
1. Hold down the Control key and choose Help > About iTunes (Windows) or iTunes > About iTunes (Mac).
2. Release the Control key.
3. As the iTunes and QuickTime version information scrolls, you will see the last connected iPod/iPhone/Touch.

 

Good luck. You can also go to : http://www.apple.com/support/manuals/ipod/ and click on the "Upload the serial number of my last connected ipod" button at the bottom of the page.