Where do we find pure water in nature?
By pure, I assume you mean water that is good enough for drinking, that is potable water. Pure water simply means water that contains nothing other than water, which is really difficult to find in nature, since water is a very good solvent (and hence dissolves a lot of materials into it). The only places where we can expect to find water that is "pure" enough is where it has been untouched by humans. We have polluted most of the surface water bodies and many groundwater sources. The air pollution means that water vapors also carry part of the pollution. The only remaining options are the glacial ice and pristine water bodies. The glaciers contain ice that is yet to be polluted and pristine bodies also contain pure water since they are away from human intervention. These are probably the only sources (apart from water buried deep into the ground) in nature where we can expect to find pure water. Still these waters may contain naturally occurring minerals, so they may not be "pure" in all senses of the word.
Hope this helps.