You can buy it on eBay, Ave probably other places online. You can buy it at most car detailer shops. That's what it's designed for, car detailers, which is why you don't see it at many consumer retail locations like Auto Zone or Walmart.
To apply, you crack the glass vial contained within the applicator and wipe out evenly on the window. It goes on like a very thin oil. You let it sit for a minute or three and then wipe it off / polish it with a paper towel. Because it's this very thin oil-like substance it doesn't want to wipe off and dry like water or window cleaner, so it takes more wiping than you'd think. Your basically polishing it into the window, but it's not nearly as much work as, say, waxing and polishing a door or fender.
On the Sprinter I apply it to one side of the windshield, then do the other side. By the time I'm done with the second side, the first side it ready to be wiped/polished. Then I wipe/polish the other side.
You want to clean the windshield really well before applying the Aquapel, so including cleaning and applying and polishing, it's probably 15 minutes.