HGEs are something I weave in to just about every play session. I'm not one to hunt specific mats, instead choosing to gather what I find and trade for what I need. So I'm the wrong person to give advice on finding specific ones, where the system states play in to it, such as famine, outbreak and the like.
I have a Krait II outfitted for combat sweeping and HGE-Hunting. Something like a cargo rack and a class 3 collector limpet controller are really all you need. Any limpet controller will do, but a typical HGE will only have five or six examples of the mat, so no need to go overboard.
There are two ways I find HGEs, either the nav beacon or the FSS. Nav beacon is faster in systems with many signals, but you gotta fly in to it. Either way, you want to find all of the signal sources and then all you need to do is fly to the HGE. Once you drop you can check your contacts to see what's there. Scoop or limpet them up and repeat.
The HGEs are on a timer, and can be far away. You have to enter the USS before the timer is out, but then you're free to gather at your leisure. I see lots of threads on these boards complaining about HGEs. I reckon if you're looking for specific mats it can be frustrating. But if you just take what you find and trade for what you need it works great. I find HGEs in almost every system I look.