Unlike many sugar alternatives on the market today, tagatose is not artificial. It was not discovered by accident by a chemist working with coal tar, or frankenstein-ed together by inserting chlorine molecules into sugar. Tagatose is a rare sugars because it occurs in nature but only in tiny quantities.
Tagatose is found in apples, some grains, and the cacao tree (from which chocolate is derived). Unfortunately, the amounts are so small that it would not be sustainable — environmentally or economically — to try to harvest tagatose from those plants.