Take a look at this fm spy transmitter which can be used as a bug transmitter or as diy fm bug too. It uses a single IC 74F13, one coil, a capacitor, one trimmer, one resistor and ofcourse one electret mic. This spy bug is so easy to build and can give you a total coverage range of about 3 to 5 meters. You may want to use a small 100mW amplifier to give a little extend transmission range so you can spy from other location.
Please use only 74F13 because 74S13 and the other ones does not work in 88-108MHz range. As antenna you can use a copper wire (>=1mm diameter) and as power supply 4 x1.2 / min.1200mA acumulators which will work for at least 7 days. Use short terminal for components and a good pcb.