Another great detailing site is
www.pakshak.com - they've got tons of good products there, including some hard-to-find brands. The off-the-shelf clay kits are fine to use. You might find a Concours-quality clay kit somewhere online that is reputed to be the best thing out there, but I've used store-bought kits for years and have never had less than a great result from them. Don't worry about buying one at the store and wondering if it's sub-par.