What is the criteria for aftermath locations to spawn? Is it random or does the system have to be in war?
Civil War and War States are more likely to produce those, but I have gotten them in normal state systems too, just less frequently.
Basically, if you see Conflict Zones on the Nav panel, you're in a good system for those and will see these spawning in USS pretty often.
As for Convoy Beacons not having anything in them? .... I've never had that problem myself, and I use this technique frequently. Its either a bug with your instance, or the beacons are either in the process of ramping up to start producing ships in them, or they are done and just have not been removed from the system yet. Waiting around for 5-10 minutes might be worth doing next time, just to see.
Speaking of Conflict Zones... I have found most of these same materials in the debris left from destroyed ships in conflict zones. Its obviously more dangerous trying to farm those with everyone shooting each other all around you, but if you use some good long range collector limpets it can be done effectively. YOU need to destroy the ships though. No debris spawns from ships blown up by NPCs.
The last time I needed Chemical Manipulators and Conductive Ceramics I found multiples of both in a CZ from a T9 and Clipper I blew up. Just make sure you PICK A SIDE! Shooting at any of those ships without being with one of the two factions will turn ALL of them RED against you.
These days, I always make sure I have some long range collectors onboard if I am going to be doing any CZ work, and when the situation looks safe to do so, I will stop dead and try to grab some of these mats while there is a lull in the action. I figure I am there anyway, why not get some hard to find mats at the same time?
(Debris tends to drift quickly in CZs, so having long range collector limpets increases your ability to grab these things without having to fly right up to them and chase them along)