Hi Grant,
Rainex is available at most auto stores. Try Autopro!
It goes under the military term of Rain Repellant RRII, (yep is has a NATO stock number too!) It is alcohol and silicone mixed together, you rub it on a windscreen to allow it to dry.... water then beads off. It is great stuff. I have used it for about 12 years on all my vehicles..
There is another product which is less effective, but claims to be the same.. It aint! ie. don't buy it!
http://autospeed.drive.com.au/cms/article.html?&A=0603
Ian