There isn't any. NGUI is a UI system, not a sprite animation system. You can do some basic sprite animations using UISpriteAnimation, but NGUI is really not intended to be a full-blown sprite animation solution.
P.S. You could set up your own very easily btw -- use a UITexture to draw a sprite sheet texture, and have a custom script set up the UVs based on what you want them to be. It's a couple of lines of code.