I needed fragments for engineers, so I followed your advice. Reading the (possibly out-of-date) wiki it sounded like you need a thargoid sensor, a thargoid probe, and a thargoid link. The link can be obtained at the site but you have to bring the probe and sensor. So, armed with a thargoid sensor scavenged from the wreck of the titan, I set off for the Pleiades.
Finding a probe turned out to be quite difficult. Before the war they were easy enough to find, but not any more. In my search for a probe, I found enough sensors and sensor fragments that I just went ahead and unlocked the engineer without ever setting foot in a thargoid structure.
But just now (after spending an hour dropping into signal sources while looking for a probe) I dropped into Non-Human Signal Source [Threat 5] in my puny exploration-outfitted Mandalay. Among the wreckage of ships just destroyed by a thargoid was a probe! As well as the thargoid that just destroyed that ship for carrying the probe! And I am NOT in an AX ship, nor a sub-20 cold ship. So I saw the probe and said "YIS!", the thargoid saw me and said "BLOOOOOOORRRRRR!" and started intercepting. I don't know my goid ships, so I don't know what its weapons are but it was big (most of them are). I put full power to shields and tried to scoop the probe before destruction. The goid closed the gap too fast and was between me and the probe, charging its weapons, so I had to run, leaving the probe behind. It gave chase, I turned around, reversed course and shot past it, so that now the probe was ahead of me and the thargoid was behind me. I raced towards the probe, the problem was that if I went too slow, the thargoid would get me, but if I went too fast, the probe would be destroyed instead of collected by the cargo scoop.
A few seconds of ace piloting later, there was a probe in my cargo and a thargoid losing distance in the rear!
Next step: is there a good in-game way to find a thargoid structure? Eg. a listening post somewhere that starts you on a breadcrumb trail of clues to follow?
Having gone to this much trouble, I think I'd like to maximise the adventure rather than look up a location online