i will probaly buy my water since i have a 17.090907 gallon
I always recommend using RO/DI (at least RO) water. If you look at
Columbus's water quality report (PDF), you'll get an idea of the stuff that's in the water which you don't really want to add to your reef. On top of these things, it's highly advisable to mix your saltwater with pure water so you'll get predictable mineral levels (e.g. calcium, alkalinity and magnesium). Keeping things stable for your corals will be more difficult using tap water.
When asked whether to get an RODI filter or not, I usually use 50 gallon tanks as the breaking point where you're going to be carrying so much water to/from the store that you really will be wishing you had a filter at home (that would likely be making better water).
At ~20 gallons (you), I would say just to use water from the LFS if possible (preferable by far to department or grocery store water) until you can save up for a filter for home. Around $1/gallon seems to be the going rate - a little more if you get pre-mixed saltwater.
As someone who used to schlepp
a lot of water, I would also not recommend skimping on buckets if you're not making the water at home. A full bucket of water weighs in at around 40 pounds after all! At least make it a little easier on yourself by having one or more of these:
2" Opening 5 Gallon Carboy With Molded Drain | U.S. Plastic Corp.
At $12 they might seem a little pricey, but they are so much nicer than a bucket for transporting water - and in the long term of a reef tank $12 is practically $0. Totally worth it. The beefy, molded handle and rounded corners are two of my favorite qualities and are really what make is my first choice for carrying water, but the multiple pouring options, a drain valve that accepts 1/2" tubing and a standard 2" opening are all features that add a ton of value and potential uses to this vs. a standard 5 gallon bucket.
Good luck!
-Matt