Most "bugs" or listening devices are going to be wireless, unless the building owner or occupant puts them in. Therefore, they will have an RF transmitter. Typically you would turn everything in the room off, as a lot of electronics put off some RF noise, then go around the room with the detector. The bugs will be very low power, so you pretty much have to sweep over everything fairly close.
I don't know anything that will pick up a hard wired microphone.
Hidden cameras can be done the same way, if they are wireless. Hardwired cameras are harder to detect. Usually it's done with special device and you are looking for a reflection from the optics. Not very easy or reliable. You wouldn't see the reflection from a pinhole lens, or too far off axis on a standart lens.
Anything else you're getting into high dollar specialized equipment, and then it depends.
Can you be more specific in the type of equipment you want and the budget you have as to be honest you pay for what you get!
There are some decent spy shops around on the net as mentioned but i dont know whether they sell effective TSCM kit as a keyring bug hunter will not turn up much, if youre looking for top end kit then look no further than shearwater ECM and speak to John. The monocle that was mentiioned earlier which is used to locate covert lens is quite useful and fairly price realisitic.
I hope you find what youre looking for and Im sure the group can help where needed...