Kosher salt (or koshering salt) is a more American-known name for what we in Britain call flaked sea salt. It's not jewish or anything like that, it's just the kind of salt they use in the koshering process to draw the blood out of the animal. The difference with table salt (as explained by Alton Brown) is that sea salt is more naturally grown (like a wheat crop), and harvested rather than manufactured, and forms hollow pyramid shapes. These don't need any added ingredients (eg. desiccants) to stop them clumping, and you often don't need to use as much salt as you would do with table salt.
After watching practically all of the Alton Brown cooking show 'Good Eats', I've invested in a salt cellar/pinch pot similar to the one he uses and some Maldon Sea Salt Flakes. I bought a small box to begin with to make sure it was the same as the Diamond Crystal salt, and it is, so I've now bought a bigger box.
You'll find Maldon Sea Salt Flakes in the majority of british supermarkets (Tesco, Sainsburys, Waitrose all stock it), alongside the spices, usually on the lower shelves. The Maldon site also has a stockist list if you're interested.