In order to sell your music on any digital music source, not just iTunes, you'll need a UPC code assigned to your release.
What's a UPC code, you ask?
UPC stands for “universal product code”. It is a unique code for your record or any other merchandise. You've seen a million of these over the course of your life. They are the bar codes that are on the back of anything you buy at the store!
You have a few options when it comes to obtaining a UPC, and they're all about the same price if you know where to look.
One option is going through your CD duplication company. For a small fee (between $20 and $50), you'll be assigned a UPC code for your product, which you can use on both your regular CD and your digitally distributed version. Just ask them, if they haven't offered it already.
Another option is the great service CD Baby . They're an online store that's a major player in the digital distribution market. They'll assign you a unique UPC for $20. You can also do a Google search for "UPC Code", and you'll get some great results -- don't pay more than $50, though!
Now you have your code, you just copy it down and send it to your mastering engineer. He will then embed the UPC code in the metadata of your record and finish up the process for your digital release!