Use this tool to locate G5 materials. Here are listed HGE detections by EDDN commanders since the last BGS tick.
If you can't find signals when you jump to a system, reset signals by jumping to another system, and jumping back.
You can "re-use" an HGE by exiting the game completly (not just going back to Main Menu) and log in again. Jump to supercruise boosting with the throttle at zero and drop to the USS that just popped behind you.