Here is the most stable, smallest, and energy saving FM transmitter. It uses only single transistor oscillator, PN2222A. Other high frequency / low noise transistor like 2N2222A, BFxxx series, BC109B, C, and even well known BC238 could work as well. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor. FM Frequency is determined by a parallel LC resonance circuit and shifts very slow as battery drains out. Its max range about 30 meters max at 1.5 Volts.
Main advantage of this simple FM circuit (below) is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places.
Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. This circuit is especially valuable in amateur spy operations.