Google for your local store.
Click on the store, and you will see a link beneath the iPad 2 for reservations. Stock comes in daily and to stand a good chance of picking one up, you will need to login at around 9pm each today to reserve your iPad! Best of luck, I got mine today :-D
e.g. for my local store, Apple Bullring Birminghamhttp://www.apple.com/uk/retail/bullring/