Use google and type in 'icon library'. This is one of the hits.
http://www.rw-designer.com/icon-library
It seems to have a great set of icons.
And if you type in 'icon grab' you will find programs that will allow
you to grab and extract a single icon from a dll file. Not that you
have to, but some people wish to grab a single icon, then edit it to
tweak it for their own use. You can't directly edit a dll icon, or
not that I know.
http://www.download3k.com/Photo-Graphic/Font-Icon-Tools/Download-TM-Icon-Extractor.html
This is just one of them.