For a smaller system purchasing distilled is an option. If you have a LFS with RO/DI available that is also an option but make darn sure you have them test the TDS or conductivity of the water before you but it. Too many shops let there maintenance go and the TDS suffers.
Yet other options are bottled RO water as long as it is not labelled for "drinking water" and has minerals added back in for taste, or a Water & Ice type store where they have a live person on staff to test the water for you. I have found they bend over backwards to show you their water quality and systems once you tell them it of for a saltwater tank or reef. Its RO only but they usually do a great job and TDS is very low.
I have a 10 gallon nano so my water changes aren't very large. I've been using walmarts distilled gallon jugs for auto top off water and water changes with no algae breakouts for over a year.
In the long run go to the sponsors here and get a RO/DI system of your own, it will pay off for your convenience in the long run, you can have water ready at any time if needed, if you are in this hobby for a long time, serious about it, that is the only way to go.
The nice thing about an RO/DI system is you can use it for drinking water and your ice maker too so the purchase becomes must easier to justify. We have owned a RO since 1990 and RO/DI since 1994. Paid for itself many times over!