This is what the Apple HIG has to say about icons. You should read it if you ever use icons. Even if you are not drawing your own icons, you need to understand how they should be used. (eg: icons in a toolbar should have a flat “head on” perspective, not the three-demensional look they have in the Dock.) You’ll find the icons you need faster if you know what they should look like.
Websites with icons you can use freely: IconDrawer, Iconfactory, Kombine.net.
SystemIconViewer (source included) by Noodlesoft is a useful tool. It lets you browse over 100 standard OS X icons that are available programatically.
For getting paths to private OS X icons, try poking around inside CandyBar.app — A commercial program that lets you customize just about any icon on your system. As of v2.6.1 /CandyBar.app/Contents/Resources/English.lproj/IconData.plist contains information on where icons are located. Icon locations do change completely between releases of OS X, even if the icon itself does not! I found CandyBar to be a better source of up-to-date icon locations then google. IconData.plist is pretty big and dense, but you can search it for keywords if you open it in Xcode, which helped me a lot.
(Although I haven’t used any of them personally, these are some design firms Apple recommends, if you have the cash.)