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)
Amazon Policies and Agreements
Amazon Services Europe Business Solutions Agreement
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: