Free floating UAs spawn in systems that are between 135 LY and 150 LY away from the star Merope in the Pleiades nebula. This know as the UA shell, and has been around since 1.4 or so. Within the shell, they spawn randomly, but some systems have better spawn rates than others. The ARIES DARK REGION systems are mentioned as the best spawning grounds, and my experience in ARIES DARK REGION GW-W D1-52 was that I needed no more than 5-10 minutes to get one. Circling the star at 100-200 Ls distance is the recommended tactic, but you need to be flying. Lurking in SC at throttle 0 no longer works.
Dirty deeds done dirt cheap for reasonable prices