Google is a great search engine, even for torrents. It’s a really old trick, but it can be quite useful sometimes. Just add “filetype:torrent” in google before your keywords and google wil find the torrent. So if you’re searching for a “harry potter” torrent you should type something like this: “filetype:torrent ubuntu”.
The filetype command also works for a lot of other file types, obviously. In addition to torrents, you can easily narrow down your search results to .PDFs, doc files, and many others.