Distilled water will work. Avoid mineral, spring, all natural water or fitness. Buying water like this may look cheaper short term but it really cost a lot long term. I'm speaking from past experience. I would bite the bullet soon and get a RO / DI when you can afford it.
Shop around look all over the net, news paper buy and sells, you'll find a second hand unit pretty cheap, just add new filters you'll be on your way in no time. Bottle water will cost you more in the long run, and theres way to many minerals in most bottle water anyways.