I actually have started new tanks with tapwater, figuring that the muck from the fresh live rock would pollute the new water anyway. Many people will say that is absolutely wrong, and clearly it's best to use the best water. At any rate, you are going to have to make considerable water changes etc in the beginning and if you are going to purchase an RO system, it might as well be now.
Watch what you but from the grocery store- bottled drinking water isn't necessarily any better than tap water. Also, pet stores frequently sell RO water by the gallon, and you can bring your own 5-gallon bucket.