Unless you are in an area with serious air pollution (meaning you need a filter mask outdoors).your local rain water should be fine. Don't worry about rain as a disease pathway. In the wild orchids are rained on all the time. You can collect rain water in any clean plastic container. Rain collected from a downspout from your home's gutter system should be fine too.
I store rain in clean containers such as plastic milk jugs that have been washed and rinsed first.
Posted via Mobile Device