Ask HN: Building price comparison website

I'm building a very simple price comparison website for textbooks. Does anyone who has built a price comparison site have tips for the implementation? At the moment, it seems like I will have to query the four textbook vendors' website each time a user does an ISBN search on my site. I'm not sure if that is sustainable or the best way to go about it. The alternate is a couple vendors offer their inventory data updated nightly but there are also a couple that don't. Appreciate any pointers!