Tap water is usually the most recommended water for your frogs but should always be provided after the use of a declorinator. Although airing water for 24 hours will remove chlorine, you still need to find out what your water treatment plant is using for a chlorine type chemical. There are chlorine alternatives that these treatment plants use that are not removed by airing what so ever, so the declorinator makes that type safe.
As for rain water, you could collect it and use it but I myself stick with tap.