Error message when buying songs
Here's how to find the way to contact Apple:

Go to apple.com and click Support.
On the Browse Support section on the left, click iTunes.
Click Music Purchases in the Get Customer Service list that appears in the middle.
Click Music Purchases in the sublist that appears.
Click the Email Us button that appears.

Here's a direct link to the form to fill out to contact them for a iTunes Store music purchase issue.