Borax can be found in nature or can be synthesized from boron compounds.  In nature, it is found in dry, mountainous regions.  During rainy seasons, runoff from the mountains contains a high levels of boron.  This runoff collects in lake beds called playas.  The playas evaporate quickly due to the dry climate, leaving behind the boron.  Since there is so much evaporation, a lot of boron is left in the valleys.  Eventually there is enough boron to produce crystals of borax and other boron compounds.  The main borax fields are located in the southwestern United States, around California.  About 50% of the world supply of borax comes from there.  Other places include the Atacama desert in Chile and Tibet.

Atacama Desert