I don't know much about this, so I started by doing a web search for the phrase "Intranet Search." I saw mention of products such as HyperOffice, ChartSearch, Blossom's InSearch, Ultraseek Server, Reflexion, and Dieselpoint. So there should be a variety of options.
That said, I recommend that you do research before you decide on a product or a service. Find out what exists, as well as what customers think about quality.