Tangut Empire TARA

Is this an ancient Tara thangka come from Nepal or Tibet?

No, both answers are wrong. 

This thangka came from Tangut Empire (Xi Xia). It might be painted by Tibetan or Nepali painters in 11th or 12 century AD, commissioned by the Tangut donors. The Tibetans called them, Minyaks. They were once followers of Kagyu tradition of Tibetan Buddhism.

This beautiful Tara thangka is currently housed at Asian Art Museum of San Francisco.