Just buy a jug of distilled water from the store. You already spent 20$ on the antifreeze, just spend 2$ more for the proper water. You can even get de ionized in the auto store, but just regular distilled is fine, if your tap water is hard.
In this zone, we get our tap water from Lake Superior, which is very low in mineral content. In my experience, the rad falls apart before it gets clogged. I just replaced 3 x $100 rads, after 15 years of service, due to salt corrosion. The copper fins were thin as tissue paper, and many areas the fins had disappeared. Amazingly, the rads still worked (engine operating temperature still fine!)