In the "old" days, I'd just buy a gallon or two of Prestone at Walmart, and mix to the proper ratio with Distilled water. But as the metallurgy in cars has gotten more complicated, and as I've acquired more cars, I decided a few years ago to either buy factory fluids from the Dealer, or the exact equivalent from my buddy who runs a race shop (with access to professional after-market substitutes).
Putting the incorrect fluid in your radiator, brake system, transmission, power steering, or engine oil can be disastrous (to your car and wallet). Yes - buying from the Dealer is more expensive ; Yes - paying for anti-freeze that is already diluted to a 50/50 mix (as Dealers often sell) pisses me off as a waste of money - but in the big scheme of things you know you're getting exactly the correct fluid recommended by the manufacturer. And the Parts Guy will gladly tell you the correct amount to buy and even recommend "extra" stuff like gaskets.
The fact you are unsure of the cooling system capacity (did you look at your owner's manual) and unsure of the mix ratio tells me you may be new to auto maintenance. Please Understand - I'm not criticizing here (it's great you want to do this yourself - so few people have any such skills these days) -- my main point is that you have no room to make a mistake - you must be 100% correct with fluid replacements.