I started out by purchasing RO water at my lfs. You carry in empty jugs, they fill it up for you. But it didn't take long to get tired of this method, so I bought my own RODI system. They come in a wide range of prices, but expect to pay a minimum of $150 even for a simple one. But the convenience of having your own RO water at home, readily available, and having the peace of mind that you know what's going into your tank, is priceless.