All knol content has been discontinued as of May 1 2012. Knol authors had the option to migrate their content and set a redirect to a new location, but this wasn't done for every knol.

The best way to find the same, or similar, content is via a google search - you may search on the knol title, keywords, or author (if known) and use the results to narrow your search for the relevant content.  Over time, even those knols that were migrated somewhere but have no redirect should be re-indexed and therefore able to be found via a search.