You might want to take a look at Prosperent actually. They do exactly what you are looking for through their API and their TCPA ad system. They have access to over 50 million products from several thousand merchants just like Amazon, but they can deliver ads and products based on keywords just as you described. I setup a site using their API where I do price comparisons. The API returns a JSON response with all of the product data, you just send over a keyword for the search. Really intelligent system from what I have seen thus far.