What is the best way to get this type of water?

With a reverse osmosis system! This system usually has a particulate and carbon filter (organics, oxidants and particulates are reduced); and a membrane (reduces the TDS by about 90% -- including hardness, sodium and others as well). With an RO System, whatever impurities were in the water are typically reduced by 90% or more, leaving only high quality water behind, which is perfect for coffee.