Can I use rainwater, snow, aquarium water, or dehumidifier water on m
In short, yes.
Rainwater or melted snow is excellent for orchids, but if you have an area which is prone to acid rain, a ph test might be in order. Also, if you have a new roof, collecting water which has run off the roof may not be a good idea to use on orchids as you may pick up chemicals from the roof. Collecting a large amount of rain water can be achieved by placing a tarp or plastic sheet out in an open area and rigging it in such a way to drain into a bucket or other container.
Water collected from dehumidifiers and air conditioning units is fine to use for orchids as it is the same as distilled water, however it does have dust etc. in it. Fine to use on orchids, don't use it on your contacts.
Water from aquariums is also used by several growers on the forum to good effect. The water has fertilizer in it already! I would remind people who are using this, however, to NOT use it if you have treated your fish with medications recently...some treatments contain copper and other elements which may be in too high a concentration for some orchids.