I've only ever found Rebel Transmissions free floating in space at the Unknown Signal Sources in clouds of cargo in a random wreck, in the systems the mission directs you too.
Though I do quick run through them, dropping out of super cruise into an Unknown Signal Source, checking whats on my contacts radar, and if no dice going back right back into super cruise.
Usually however i've found them in under 15 mins at a Unknown Signal Source while randomly roaming around in target sectors
Though one time i got the system wrong and wasted about 45 mins looking in the wrong place, so read the system names correctly, and double check.... cause curse the systems with similar numerical endings