Can someone help me find a discrete listening device...?

I'm looking for one I can hide on someone without them knowing & listen from home. Its gotta be something small. My best friend suspects her boyfriend is cheating on her, but she wants proof before confronts him. I'm looking on Amazon.com cause that's were I'd rather buy it but there are so many I wouldn't know how to choose. I want it to be good quality, but not too expensive. If anyone finds one on amazon & sends me the link I'll give u best answer. :)