For the most part I use collected rain water and to a smaller extent purchased RO/DI water. But those were already mentioned.
Something I didn't see mentioned is the water what comes from your air conditioner. It's condensation run-off so should have a low TDS though individual results may vary. I collected some from my unit a couple years ago and it was less than 10ppm. It's not a fast water source but you could get a gallon or two per day by leaving a bucket under the drain pipe.