I spawn these daily, spawned 5 for one guy and 1 for another just yesterday.
Use the DOS mission others have suggested but don't keep driving too and away from any LSC.
Instead use the method in this video.
They will spawn there from 12:00-19:00 any day of the week.
It's been reported they don't spawn till 13:00 and don't spawn on Sundays but that's false, I've spawned them as early as 11:30 and on Sundays.
You must use a personal Dubsta 2 for this to work and you will get other vehicles to spawn but 90% of the time it's a DUBsta 2.
Props to MajorVienna for first posting this video in another thred.