I am assuming you are not doing a full 5 gallon wort boil. When I do a partial wort boil (2.5-3 gallons after boil) here is what I do. I go to the store and buy bottled water. Personally I get Ozarka, but the brand is personal preference. I buy the 2.5 gallon jugs. I buy 3. I use almost 2 of them in the wort. When I first get the wort to a boil I move the last unopened one from the fridge to the freezer. I check that periodically while the wort is boiling. I take it out just as the first ice crystals are forming on the top.(usually right about when I am finished boiling the wort. I then but the almost frozen water (about 34-36 degrees) into the fermenter and then pour the wort on top of that. I generally brings the temp down to about 75-80 degrees, almost the temp for pitching the yeast. That has worked fine for me for many batches.