This is a popular question online, especially on Yahoo Answers, so I did some research and here is what I found:

You can buy an iTunes gift card at the apple store. To do this, you need a valid credit card with a U.S. billing address. You can also buy them at many national retailers, supermarkets, and drug stores, including the following:

Best Buy
Costco
Circuit City
Wal-Mart
RadioShack
Target
Rite Aid
Giant
Walgreens
Safeway
CVS

Readers, do you know of other places (both in-person and online) where you can buy an iTunes gift card? Post your response in the comments section below.