[Jeffhs]

100-mw or less FM monophonic transmitters may still be available (I used one as the FM side of a 100-mw AM radio station I had in the early '70s, before I got my amateur radio license), but I have never seen or heard of any kind of low-power stereo FM transmitter. I am not sure stereo FM is even allowed under the FCC's Part 15 rules for unlicensed AM or FM transmitters, although I can't see why it wouldn't be; after all, the FCC is only concerned with the transmitter's output power, not with the format of the signal (AM, FM or stereo FM) and, of course, the program material carried over that signal, with at least one exception. The exception is this: Even at 0.1 watt (100 milliwatts), the FCC's indecency rules still apply. Don't try to broadcast an underground radio program using all manner of indecent language or content and expect to get away with it for any length of time.