I got mine using this game (4th of July Mahjong)
Purchase at least one qualifying app offered in the Amazon.com Appstore for Android in the webstore or on an Android device. The apps that qualify are apps for Android offered in the Amazon.com Appstore for Android that display the offer message on their webstore product information pages. Qualifying apps will not display promotional messaging on their Android on-device product information pages.
After completing your purchase, you will receive an e-mail indicating that a $1 credit for Amazon MP3 music downloads has been applied to your account automatically. The e-mail will also provide instructions on how to redeem your credit.
Promotional offer limited to one promotional credit per customer.
Promotional offer is valid from July 2, 2012, through September 30, 2012, and subject to change. You must redeem the credit by October 31, 2012.