Ok... 😉
Icon sets can be found on many sites... first place to look is using your own Apple menu -> Mac OS X Software... There you can find a whole category of "Icons, Screensavers, etc."
To change them there are several 3rd party software to install icon sets, but if you just want to change a few, including the HD icon, then it might be easier to do it manually:
Select the file/folder with the icon you like and choose "Get Info" (from the File menu, right/ctrl-click or cmd-I), then click on the icon (it gets a blue shade) and Copy (from Edit menu or cmd-C). Mark the object you want to change the icon of, open the "Get info"-window for that, click the icon and Paste (using the Edit menu or cmd-V).
To remove a custom icon, just open "Get info"-window, click icon and press Backspace...
Enjoy... 😉