Lot's of places sell it Rob. Autobrite Direct, CleanYourCar to name but two.
Polish with SRP, pretty much every 6 months, maybe more. Personally I'd time it with applying the wax, given both 476s and 845 will give around 6 months of protection if applied properly.
In terms of applying, THIN is best. Lightly dampen the applicator pad, and apply in tight small circles to ensure even coverage. Do two panels, then check the first by doing a finger swipe. If the panel comes clean, buff it. If not, allow a little longer to cure, check again then buff when ready. Both are as easy to use as each other provided you use them properly, but if you're new to it, you'll likely find the liquids a bit easier. Just don't over apply.......you have been warned

It's a nightmare to get off if you let it over cure when over applied.