If you know people who have to run dehumidifiers in their basements, they should be able to supply most of the water you need for outdoor plants. Distilled water is the waste product, so the cost is free, other than for the time to collect it. I can get a couple gallons of water per day out of mine in midsummer. I have a 7.5 gallon covered tub that I dump the water into and I fill plastic jugs from that. I used to have 20+ gallons of storage to bridge dry periods and I thought about getting a drum or two. The process works fine until the dew point drops in September and the basement gets dry again. But the outdoor water demand drops at that time too, so it isn't a huge problem, especially if you can store a lot.