You can search the contents of packages included in the various Ubuntu releases on the Ubuntu Packages website. Look under the heading "Search the contents of packages".
For example, here are the search results for libnss3.so in lucidfocal (1020.04):
http://packages.ubuntu.com/search?searchon=contents&keywords=libnss3.so&mode=exactfilename&suite=lucid&arch=anyhttp://packages.ubuntu.com/search?searchon=contents&keywords=libnss3.so&mode=exactfilename&suite=focal&arch=any
