You can find your Amazon seller agreement if you log-in to your account and go to Account Settings Reference  Policies and agreements Amazon Services Business Solutions Agreement.


You can also use the following links to go directly to the agreements:


Amazon Services Business Solutions Agreement (the US Version for US marketplace)

https://sellercentral.amazon.com/gp/help/1791?language=en_US&ref=xx_1791_cont_521 


Amazon Policies and Agreements

https://sellercentral.amazon.com/gp/help/help.html?ie=UTF8&itemID=521&ref_=im_521_bred_1791& 


Amazon Services Europe Business Solutions Agreement

https://sellercentral.amazon.co.uk/gp/help/external/201190440?language=en_GB&ref=efph_201190440_cont_201190440 


Amazon doesn't offer the option to download the agreement and the tax office require the agreement to have your business details included. You will need to copy the agreement from Amazon's website and paste it into a word document, then include your company details in the header of this document like in the example below: