You can find icons at websites like Interfacelift:
http://interfacelift.com/icons-mac/categories.php?id=9&page=1
Changing them can be a little tricky if you are new to Macs. Here is a little instruction:
1) Download the icon. Make sure it is has the extension .icns
2) In the Finder, navigate to the app whose icon you want to change and select it.
3) Right-click on the app in the Finder and choose "Show Package Contents".
4) Open the folder there called "Resources".
5) Find the .icns file that contains the standard app icon. It's usually called app.icns.
5) Rename the new icon you downloaded to match the name of the one in the Resources folder.
6) Delete the app.icns icon from the Resources folder. You'll probably need to enter your administrator password.
7) Drag the new app.icns file into the Resources folder.
8) Drag the old icon out of your Dock and drag the app back into the Dock and the new icon should appear.
Basically, you're switching out the icns file in the app's Resources folder with a new icns file.
There is a simpler way that may or may not work.
1) Download a new icon, icns or png.
2) In the Finder, navigate to the app whose icon you want to change.
3) Right click the app and choose "Get Info".
4) Select the small icon that appears in the top left of the Information window.
5) Drag your new icon onto the old one, and it should change instantly. However, I've been having trouble with this method in Snow Leopard and even in Leopard it didn't always work.
