It's the question of the day!
If you're trying to find an NAICS code, try using the official government website at http://www.census.gov/eos/www/naics/. This site has a really useful search and, although it's not a library database, it is a reliable information source.
If you need more information about NAICS, such as correspondence tables to see the differences between revisions to the code, try this site: http://www.census.gov/epcd/naics02/.