Where can I find Android's default icons?

\path-to-your-android-sdk-folder\platforms\android-xx\data\res

you can use

android.R.drawable.xxx

(use autocomplete to see whats in there)

Or download the stuff from http://developer.android.com/design/downloads/index.html

You can find the default Android menu icons here - link is broken now.

Update: You can find Material Design icons here.