I want to make an default Alert Dialog with a small modification. Where can I find an xml layout file of default AlertDialog?
All default layouts are included in the Android SDK. In your SDK installation, see the following:
<SDK_INTALL_DIRECTORY>/sdk/platforms/android-18/data/res/layout/alert_dialog.xml
(or replace "android-18" with the API level you want).
Note that you will only see API levels that you have downloaded via the Android SDK Manager.