Rainylotus, you might try using a site such as
http://www.ereaderiq.com/ They don't sell ebooks directly: but you can search by author or title to find retailers that do sell the books. Most importantly for you, you can set a search for author or title, and set an acceptable price range, and ereaderiq will email you if and when that book goes on sale at any retailer they track. It's certainly helped me add ebooks to my collection that I couldn't afford otherwise.
If by any chance you use Calibre to handle your ebooks, I think that program also has a feature to search available bookstores, and I think they cover a wider range of stores. As far as I know, however, you can't set it up to notify you of price reductions.
KentE