Popular search engines like Google and Bing search vast amounts of data, but they have a catch some people aren't aware of: They only search HTTP sites. There is a whole separate world of file storage out there on FTP (File Transfer Protocol) servers. You cannot search for FTP files on Google or Bing, but you can using FTP-specific search engines.
Search for FTP files with FileSearching.com. This website offers a barebones interface similar to Google's. You simply type in your search term and click "Search." You can also customize the search by file type (image, audio, video, server) or URL extension (.com, .org, etc).
Search for FTP files with NAPALM FTP Indexer (searchftps.com). This site doesn't offer as many filters as FileSearching, but it does let you better organize the results. For instance, you can order them by date or by size.
Search for FTP files with Search-22.com. This site compiles multiple FTP search engines, making it a good one-stop location for multiple searches. The downside is that you lose the individual search engine's distinctive features, like filtering by file type or organizing results by date.