I got mine at Best Buy (DON"T GO THERE, they rip you off on price). I have the Cobra ESD 9550 and it works great.
http://www.shopcobra.com/product.asp?sku=1756124 It goes off and the cop is about 1/2 a mile away. Just keep in mind that this only helps for 1 out of 3 ways they can catch you for speeding.