what parts are needed to build a listening device and where can i buy them?

Answer 1

The expression "listening device" is a bit global....
What do you want to do with the signal? -
1) Just listen to it in another room, same house?
2) Listen to it some distance away?
3) Record it at site, listen to the recording later?

If 1): I wouldn't bother to build one myself. Get an "electronic babysitter" or an internal intercom which comes about as cheap as buying the parts. A good alternative is also a so-called FM Microphone or wireless microphone, often used in Karaoke amplifiers. They send out a true FM radio signal which you can tune into on a normal FM radio (but so can all your close neighbours!!).

If 2) I don't think you would know how to build that yourself (judging from your question). Because it is unlikely that you can run a pair of (undetected) wires over a longer distance, you need a wireless radio to transmit the signal, and to build that is for experts. In this case, why not buy a pair of simple Motorola Walkie-Talkies, and use a rubber band to keep the "listening device" on constant transmission. But careful, a lot of other people can then listen, too....!

If 3) Again, why build, there are cheap cassette recorders on the market. The problem is they are too big to be "invisible", they can make a noise (especially when the tape is full = switch off), and the recording time is limited to max. 1.5 hours.

So, give it some thought whether it is all worth it to spy on your parents or older sister.....!