How am I supposed to get an etherial?

[Moltenore]

I went through the main plot, captured Alien Orbital Stantion, even got an etherial keeper alive. Now, to proceed, I need another etherial (soldier, lider, commander, sentinel, speaker, knight or Grand Master) to unlcock Alien Neural Network Prerequisite, but I there are no on alien aircrafts, nor in alien bases, and, as I see, there are no further scripted missions with them neither. The game is getting boring, so I want to proceed ASAP.

[Solarius Scorch]

Keeper already unlocks this prerequisite, so you shouldn't have to capture another.

[Moltenore]

That's right, but I can't interrogate Etherial Keeper before I get another etherial among that ones, which I listed higher.

[Rag]

I had this same issue, there are a few options I found.
1: If you haven't finished the Zombie storyline yet, try to capture the boss of the final mission alive as it has Ethereals as a bonus topic (I missed this chance myself but would have saved me a few months)
2: can wait until Ethereal missions, I personally did this but it took a long time, I had Ethereals base retaliate against me and I let them find me so I could capture them.
3: can gamble on the mixed alien crew missions, I found some ethereals are joining some random UFOs in 2000 not sure at what date they begin to appear.

And yeah the Keepers do require Ethereal topic already researched before they can be interrogated, not sure if that is intended or not as it did seem to give me a major roadblock for a few months. This time went past quickly IRL since I was at the point I could just intercept and do a few missions to keep score positive as I finished the other storylines and didn't need money anymore. So at least you can use that time to finish up loose ends as you wait.

[Solarius Scorch]

Ah, right - any Ethereal is needed first. It was done so for technical reasons.

I didn't foresee a problem with getting basic Ethereals, but I will address it in the future.

[zee_ra]

I went through the main plot, captured Alien Orbital Stantion, even got an etherial keeper alive. Now, to proceed, I need another etherial (soldier, lider, commander, sentinel, speaker, knight or Grand Master) to unlcock Alien Neural Network Prerequisite, but I there are no on alien aircrafts, nor in alien bases, and, as I see, there are no further scripted missions with them neither. The game is getting boring, so I want to proceed ASAP.

I found that a first capture of a golden ship yields in general about 3 captives.  At that point, the X-Com should already have access to strong Psi, as well as small launcher.  Either one is enough to enable serious captures.  The only difficulty, frankly, is with the initial couple of turns.  The basic tactics with smoke screen, even through the mortar fire (the smoke cloud is larger from dedicated smoke mortar shell explosions) directly from the vehicle, don't always work.  In such cases, there's little choice but either potentially sacrificing a couple of robots, or sacrificing a handful of aliens through the use of guided missiles (even an improved launcher ones are more than adequate for these purposes).

I agree with you that the opportunities to capture Ethereals are quite rare in this game.  Therefore, the best tactics seems to maximize the capture at the initial golden ship encounter.  It is possible to do this in casualty-free fashion, especially with a crew that has AI units.  Ideally, the units would be using the power armor robot shell, but in practice it's not always possible to get to that point in research in time.