so basically salave operations are the best way to fish for them?
In my experience - yes.
Best case is finding a "distress signal" POI with an intact ship with two PR slots.
Yes they sometimes are target of "recover item from wreckage/crash" missions
although it does not make much sense,
because it is an emergency landing situation, ship is not damaged at all.
I have grinded 29 working PRs in about an hour in alpha AFAIR
"For the science" naturally, because relogging is bad, and exploit, and cheating and ruins the game blablabla
"Normal" distress signal POI was sometimes expiring (disappearing) after relogging .
Mission POI can't expire unless you pick up item you were sent to get,
so you can abuse this fact if you feel like it (and if it's still working like it was in alpha).