In order to aid physical searches, below are some pictures of kit and spy-shop transmitters. All transmitters are obviously not going to look like this, but a graphic reference should help. Apologies, most of these are catalog shots.

RF Transmtters


Telephone Transmitters

Ramsey Electronics telephone transmitter  Transmitter from Carl's Electronics 
Transmitter from Advanced Intelligence. Remember, 
transmitters can be fit into almost anything.
Microkits telephone transmitter 
Information Unlimited transmitter