There are tons FM bugs all over the internet. You may find small, IC based ones, powerful, sensitive and so on and on. Take a look at this transistor based FM transmitter that actually fits in a mint box.
This FM bug uses only widely accessible discrete elements. It even uses couple general purpose BC547 transistors that can be replaced with any other similar if needed. As voice input there is an electret microphone used which is sensitive enough. The carrier signal of FM transmitter is chosen between 88 and 108MHz. To see in detail how signal is amplified and mixed with carrier signal here. Whole FM bug is powered with single 3V coin battery which is a biggest part in transmitter. Anyway it fits Minty box nicely and is able to transmit audio signal over 25 feet.