Which salt do you use is a horrible question to ask. :p It usually starts a war about which brand is best, etc.
Pick one and use it all the time. If it needs to modified, add whatever it's lacking to your pwc
mix and you're good to go.
For online retailers I seem to go with the doctors first, though I'm probably switching to ThatPetPlace.com
due to a recent change in the standard shipping at Dr.F&S. If neither of those place have what I'm looking for BigAlsOnline and MarineDepot come next along with a bunch of other sites I frequent.
The light from those windows will most likely cause you some algae problems.
Good luck with adventure. It's lots of fun.