At one point, with Citibank (Diamond Preferred Rewards) you could buy Amazon.com gift certificates and get 5% back. I don't remember whether that was on top of the 1% you'd get for regular purchases.

Anyway, if you have an Amazon.com credit card you can get 3% back. I'm pretty sure the details/links to apply are available from the Amazon.com site.
It may only give you gift certificates towards future purchases; don't remember.