Well, since people wrote you ought to get the parts even from dead bodies, can you really say I missed those opportunities if they never even presented themselves?
I can't shoot down most large ships per se - my fighters tend to get shot to pieces - but I've done plenty of large and very large ship missions. The church's sway government missions involve very large ships, for example, and I've done at least half a dozen of those. Problem is, the chuch doesn't have 'em power armor dudes, they sport matrons and maidens and altar boys.
The other large ships I did - as I wrote above - was a mercenary vessel, just loads and loads of 'em flying dudes, no power armor.
I used to get frequent raider missions in Australia, but no longer - haven't seen a raider ship in many many months now. I have not seen the Academy in years. The Guild makes an occasional appearance with a freighter, but only comes with low-level dudes.
"You have to try" is a funny suggestion when there's nothing I CAN try. For half a year, I did almost every mission, the only exceptions being a couple of two person civilian skycars and some heavy mercenary missions (as I wrote, I did the first and decided it's not worth it).
And it's not like I never interrogated anyone. I have done loads of interrogations, everything from Savvy Guys to Star Gods. I have the church's Cydonia codes and their Bible translation.
I have interrogated Government operatives and a General. I have interrogated corrupted Doom soldiers and a Chrysallid ... and so on and so forth. I may have missed some specific types, but I have done plenty, rest assured.
I have almost full global radar coverage, only antarctica and the tip of south america are missing.
Like I said, I don't know if I have what it takes to grind out months doing mundane missions that are just samey, dull and boring at this point just for a chance to maybe, MAYBE get that piece of tech that stupidly locks away all the game's progression.
What this mod would need (I realize this is strictly a pipe dream as it's beyond the current capabilities of OpenXCom and adding it no small task) would be a REAL black market. Where what's on offer is in constant flux. Check frequently, or you might miss that old fighter hull. But if you check frequently, you can get most anything if you're willing to pay. Especially power armor parts should be a no brainer. Also tank wrecks. I researched all those fancy tanks an eternity ago, but have never been able to build even one because it requires you to have a wreckage that you'll only get in specific and pretty rare circumstances. But again, I realize this is probably impossible to implement - some other solution has to be found for the mod to remain enjoyable and not force it's player into a many months long hiatus in which nothing interesting happens.