Alternative to distilled water for pH test
Rainwater might not be the best option. 

Rain water is naturally acidic due to the reaction of CO2 forming some weak carbonic acid [see here for explanation]. 



 Source: Wikipedia 

As a result, pH of "pure" rainwater is usually between 5-6; usually around pH 5.6. See here.


  Most rainwater has a pH of 5.6 to 5.8, simply due to the pressence of carbonic acid (H2CO3). 


Presence of any sulfur or nitrogen oxides in the air (perhaps from burning coal plants or city traffic from 100s of km away) would lead to rain becoming even more acidic. From environment.co.za:


  Sulphur dioxide reacts with water vapour and sunlight to form sulphuric acid. Likewise NOX form nitric acid in the air. These reactions takes hours, or even days, during which polluted air may move hundreds of kilometres. Thus acid rain can fall far from the source of pollution.