The Wirecutter recommends the I-Wave, but any Bluetooth adaptor should suffice. It has a 30-pin port on the bottom that you click onto the dock. An indicator light in the top corner tells you that it’s working.
Wirecutter also tested adaptors called CoolStream and Wave Jamr, but found I-Wave to be unbeatable for the price. Like the CoolStream, it has a range of about 7-9 metres, but it’s cheaper. And you don’t have to buy a new iPod dock, which is nice if you just bought a new iPhone 5. [The Wirecutter]