well you can buy RO water- reverse osmosis water, which is really pure at those water machines infront of the stores like albertsons, safeway, kmart, etc for a couple of cents a gallon. i pay 35
you can use tap water, just dont use water from the hot water facuet, because thats bad water, the waterheater has curd in it and of course, you dont want that in your tank. Use cold water, put in the salt, use a powerhead and use a heater to warm up the water and let the water mix for 24 hrs before putting it in.
The powerhead will mix and airate the water for you and the heater helps you from shocking the fish plus you can only tell if the salanity is right when your water is around the same temp as your tank. Basically you need this (bucket/rubbermaid tub, cold tap water, conditioner, salt, powerhead and heater) remember to put conditioner before mixing the salt like it is recommended(dont know if this is true)