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: (Updated 10/27/2011)
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.