It depends in the efforts you want to put

there are some people I know who studied or knew from stories of the old generation, all the locations where the biggest hospital fields or location at the front during WW1 / WW2 took place, and after they collected all historical proofs about those event, they just went on-side or on every nearby town/village, knocking on every door and telling they are collectors of historical items of WW1/2, like first aid kit and such

After a few hundreds knocking on the doors, you can be sure the kit of grandfather/mother who worked as auxiliary in such fields can be found in the cellar.
You can follow the best discoveries / retrieved ultra rare vintage pharmaceuticals on the reddit sub obscuredrugs. Damn, the pharmaceutical of those times was totally another world. Opioids, Barbiturates, Methaqualone, Amphetamines, and a countless number of rare and obscure pharmaceuticals - since seen the precursors for the standard pharmaceuticals were not always available during the war times, the pharma industries just made the closest functional pharmaceutical possible ;D
Dextromoramide is probably the best opioid ever made in pharmaceutical history, sadly retired only after 2 years it released worldwide, because "not safe". Yeah sure, not safe because was probably going to cause the hugest opioid panemic of the history, being a true holy grail (discovered from Jannsen, the immortal genius chemist who creared Fentanyl 4 years later as well) - with a potency of only 3.5x analgesic, got the strongest euphoric hit of every opioid (talking of the < 10x classes), and a warm body well being pleasure that get close to acetyl-f (15x).
My goal is to collect all existing pharmaceutical opioid to store in a museum, the beginning was easy, but now it became difficult if not impossible, and i'm not even at the half of the way (20 over 54, the last time I checked at least). But finding the hardest piece of the collection, an original dextromoramide vial made in 1959, was really one of my happiest moment of my life. Shame that I could get only one, so I never had the honor to try it.
Ketobemidone is another interesting one. With an analgesic potency of only 0.8x morphine, was tested as replacement of methadone to heroin addicted. After several months, the results were great - 100% of the addicted, stopped with heroin and only asked for Ketobemidone. Because it's a lot better than heroin in fact, despite the lower potency analgesic, the receptors affinity is amazing! And, even here the analogues possible, nowdays still totally legal worldwide, are just there waiting for somebody to synth them. Chinese tried but they did a huge fuckup giving us o-amkd, totally worthless opioid if not perfect replaced for amphetamine. Ketobemidone is STILL available as pharmaceutical in two EU countries, but sourcing it is a hero quest too, I spent two years before I found a source, and for a box I got asked an INSANE price. could not even enjoy it because my tolerance was already fucked up from stronger stuff.
Dipipanone is still pretty popular in south africa as pharmaceutical, and super extremely rarely in UK, I never managed to find it, and probably it will disappear completely. Main reason is just because there are no more raw materials to produce if (in a profitable way for the pharma industries). Here our chinese friend was the only time I think they did something decent, with a low potency but effective, more than o-dsmt. The problem is the half-life was SO.LONG, and acting similar to methadone mechanism, that even such apparently safe compounds caused overdoses, since people was unaware of the 24+h of half life.
The others on the list are not worth to mention, since they are either gone already or they are manufactured only on request for specific cases. Not that they would not be amazing pharmaceutical opioids tough. I heard Piritramide IV is actually not so impossible to find it - or better said, to get it if you do an OP in a german hospital and u mention you have allergy to morphine :d Liquid Tilidine is appearing from time to time to other german DNMs seller. I was around yesterday to check DNMs and I saw tilidine pills vendors, but no bottles.
So yeah, the changes of finding such rarity are only related on the efforts you want to put in. Nothing is impossible, and if nothing can be found, then it can be synthesized. Of course this is certainly not easy too, first because most chemist won't take the risk of doing a synth of a scheduled and patent pharma opioid, and secondly because any seller putting effort in realizing something like this won't make big profits either. There is no love or it's really a small Niche community who likes to hunt and research such stuff, reading about new possible opioids and trying to bring back a good RC opioid scene. All that matters if to get the strongest analgesic opioid ever and with a legal status, to make millions selling it as fake pressed or china white or whatever else.
If you check some reddit sub related to chemistry or other clearnet forums related to drugs manufacturing you can find lot of interesting things tough, recently I saw new rc opioids of brand new classes and very safe from the pharmaceutical perspective, they posted all the info and the synth route, but we talk of compounds with 0.5- max 2x morphine potency, so except people who like opioid and research them as hobby, nothing interesting for the business. And of course just because they posted or released new papers about 'em doesn't mean at all they are selling them trying to ask u get banned.
Ok, end of my useless wall ot text