Which mod is best and where to buy
As others have said, the ProVari is a PV, Mod, etc...the battery portion of the electronic cigarette.
The Kayfun is a rebuildable tank atomizer...the atomizer part of the electronic cigarette.
You could use the two things together, like using your Kayfun on your Provari, but neither does anything without the other. You need both the battery and atomizer parts to have a working electronic cigarette.
Just FYI, since you seem new, this setup will cost you probably $400+ American Dollars. You might want to consider something cheaper and less complex if you're just starting out. Something like an iStick, MVP, etc.. with a Nautilus/Aerotank maybe. The Kayfun also requires you hand make your own heating coils, which is a more advanced type of atomizer.