From what I understand rain water is almost like distilled water, no minerals in it to speak of since raindrops are formed from water vapor in the air. Someone please correct me if I'm wrong! It just seems like rain water has the potential to leech vitamins and minerals from your frogs skin in the same way distilled would.

I just buy gallon jugs of spring water at my local grocery store for like 85¢ each, that is what I personally would reccomend for your whites and your greys. I could be wrong, but there ya go, that's my 2¢ :P

Whites tree frogs can get quite large. I'm sure a 33gal. would be okay for a few months just because it sounds like that's better than what they are in currently, but you will definitely want to consider upgrading down the road.