Since Amazon doesn't accept bitcoin directly, if you have access to an account there firstly send them a request to their support asking them to add bitcoin (the same thing happened with Steam once and they finally did it).
and for now you only can do it through third parties, either finding someone who buys it for you and you pay him bitcoin. or buying gift cards. both have risks.
but buying giftcards may be better, use trusted sites like this:
https://www.gyft.com/ (check to see if they have them, i have never used them myself!)