I am not 100% convinced that specialized search engines do any better job searching for web content that standard search engines (Alta Vista, Yahoo, AskJeeves, etc.), but you might find these of interest. These may really function more like "meta sites"...just listing web pages on the subject.
eBizSearch
eBizSearch is an experimental niche search engine that searches the web and catalogs academic articles as well as commercially produced articles and reports that address various business and technology aspects of e-Business.
Search Engine Guide: Business
"The guide to search engines, directories and portals."
