Companies like Spy-Max have made devices designed for listening through walls. The devices use a microphone or amplifier to detect sounds through wood or concrete.
Retail sites, such as Amazon and eBay, carry a wide range of these devices. They can also be found at stores that specialize in spy equipment. StealthTronic is just one instance of a company that carries security equipment. The listening devices vary depending on the manufacturer and they all come with different features. The devices are portable and battery operated. The thickness of the wall that the device works with also varies depending on the specs.