Amazon.com, usually, as they have the cheapest prices I've found. It can be found on eBay, as well. List price for a single applicator is $9.99, but the best prices are the ones in the $3-$5 per applicator range. Four applicators is enough to do the windshield, side windows and mirrors.
If it's a brand new windshield, just a good cleaning with an ammonia-free cleaner (Invisible Glass, Aquapel Glass Cleaner, Sprayway) is all you need. If the windshield is more than a few months old, a good cleaning with a glass cleaner and a thorough wiping with an
Aquapel Pre-Cleaner Towelette makes a big difference in how well the Aquapel Treatment bonds with the glass. One or two towlettes is plenty. If it dries out while you are working with it, just add a little water to the towel and it'll still work.