how do i get serial number of iphone 3g from itunes

Finding serial number for a device - https://discussions.apple.com/thread/3908245?answerId=18246553022#18246553022 - from box or backup file, not from Apple.

http://support.apple.com/kb/HT2526 - You can use My Support Profile to find a list of serial numbers that have been purchased or registered with your Apple ID.

My Support Profile - https://supportprofile.apple.com/MySupportProfile.do

iTunes>Preferences(under the edit menu if using Windows)>Devices. Hoover your mouse pointer over the name of your iPhone backup. A popup will appear showing SN & IMEI number.

No box, no backup, no support profile, no receipt, no SN.

Connect the phone to your computer, click once on its name in the left panel of iTunes, look on the Summary tab in the right panel in the "iPhone" box.

I suspect the OP doesn't have the phone to do so, or its broken otherwise he/she wouldn't have said: "...unable to retrieve from any other source."

Good point. Thanks for pointing that out.