partner with Shopzilla

Have you looked into partnering with Shopzilla/Bizrate? They have a program for sites that publish shopping data which I believe would supply you with daily pricing data on a wide variety of products from numerous merchants. You could choose a slice of that data and apply your historical price tracking and alerting to it, (Disclosure: I do some work for Shopzilla as a contractor, which is why I am familiar with the partner program. Other comparison services probably have similar programs.)