Having spent far too long working this out for myself I thought I’d post for anyone searching in future! For UK folks looking to add Amazon UK to Listary’s Web Search tool, the URL you want to use is:
http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords={query}
and the suggestion provider is:
http://completion.amazon.co.uk/search/complete?search-alias=aps&client=amazon-search-ui&mkt=3&q={query}
-Tom