Dunno where you are located, but if you have good air quality when it rains why dont you just use rain water from a bucket/tank and bring it inside. Fresh rain water is as good as distilled and its $free.99. The rainiwater wont kill you your plants or your humidifier and is just as good as distilled and in some cases RO
You can obviously go and spend more of your hard earned money on filtration kits and other shit thats more than likely made in China if it makes you feel good.... but you dont have to. As long as you dont live in a city or in China, or next to a volcano eruption you could more than likely have good distilled rainwater in your backyard bucket right now. If you want to see how good your rainwater is you may also have local water testing centers that do free testings on tap and rainwater for you. I have such a free testing center by me (limited to one test a year), and my rain water is 10-40 at any given point fresh and my tap is hard (100-150) with no chlorine/chloramine or softener usage.
Mother nature can save you time and money if you learn to rub her right and respect her.
Tips on how to rub her right and get safe distilled rain water "foh free":
1. When not in use, keep your rain water bucket(s) dry, clean and with the lid on.
2. a.Check your weather forcast for rain and leave your buckets out for it anywhere from a day to an hour before the rain hits
b.when it rains check you air pollution index (on most cellphone weather widgets), low air pollution means good water.
3. After rain, examine your water, dont use old stagnant rain water that is off colored or cloudy or full of unknown bug larva, you want as fresh and as clear as possible. If the water is dirty, toss it and clean the bucket with vinegar. If it has recognizable mosquitos larva or basic green algae, its fine for your plants if used, but not for say...your humidifier or hydro.
4. Only leave your rainwater bucket out when you are due for rain, otherwise leave it in a dark area with the lid affixed
5. Clean your rainwater bucket every couple of refills and whenever contamination or cloudy/algae water takes place
6. If you plan on storing buckets of rainwater, store them in a dark place to prevent algae and keep the lids on to prevent evaporation. The lid also prevents bugs and air pollution from contaminating the water. Note that non-aquatic plants dont mind algae and mosquito larva, but your humidifier or your hydro setup might not thank you.