The best voice recorder device on Amazon right now is the Tilerec spy recorder. You need to check it out, especially while it's on sale.

In the U.S., you can pick up a mini spy (nanny cam) that supports 4K Ultra HD night vision for only $33.99 at Amazon right now, the caveat is that the deal ends in just over four hours at this price.

The Adafruit website explains a little more about the new mini spy camera which is now available from their online store priced at $12.50 and comes complete with trigger.

Then there is the Ehomful mini spy camera, which is only 1 inch and the size of a coin. The device is portable and equipped with a magnet that can be attached to TVs, refrigerators and bicycles ...

You can either go through these steps to have yourself this cut-rate spy device, or you could just use a stethoscope to listen through walls directly.