Its very easy, google "os x icons" and find sites like http://interfacelift.com/icons-mac/ where you can search and download the icons. There is a way to make custom icons but i cant seem to find it now. Once you downloaded the icon you like follow these steps.
- Select the icon you like, click file, click Get info (shortcut apple key + i"
- At the the small info page that pops up select the icon on the top left side and do apple key + c (copy).
- Now select your hard drive and do they same thing except to copy it, past it apple key + v (paste).