not sure of any premade storefronts, maybe google for something, I did run a general shopping directory 2 years ago in which I categorized shopping departments and I used links and banners from cj.com and linkshare.com which worked pretty good, I did not target a certain niche instead it was more of a general shopping directory kinda like a mall of categories, this turned out to be kinda a pain as some of those links tend to outdate themselves and need to be updated, so I think the game plan of targeting a niche or just a few categories works a lot better, which sounds like what you are trying to do, good luck