Do a search on the forums for "stethoscope" or "electronic stethoscope". You will DEFINITELY find plenty of info on this subject...trust me.
Stethoscopes are asked about all the time. To be brief, I'll mention that most people have said they're more expensive than they're worth. I think most agree that it's best to buy a fairly high quality stethoscope if you're going to need to differentiate subtle sounds. Otherwise, a moderate priced one will be enough.
That's what I've gathered from the posts here, anyway.