Diphenoxylate hydrochloride,
I agree with
Diphenoxylate/Atropine (aka Lomotil, Dhamotil, Diastop) as a good suggestion. Even though it's a mild opiate, it can be quite useful if you take enough, especially for withdrawal from stronger opiates. (Gram Parsons was tapered off heroin with high-dose Lomotil during a detox in England, actually.) And the atropine content has pretty negligible effects when taken orally, even up to quite a few pills. It's also relativel inexpensive, and I know it has some level of street popularity in South Asia, probably other places. I imagine difenxoin/atropine (aka Motofen) would be even better, but it seems much less commonly prescribed or used otherwise, either in the "developed" or "developing" countries.
Branching off from diphenoxylate, here are a few more semi-obscure members of the mild-to-moderate strength opiate/opioid class:
(Dextro)
Propoxyphene (aka Darvon) is one that used to be quite common but has become pretty obscure in many countries over the last 10 ywars, though again it was quite popular for "abuse" in South Asia as a combination product called Spasmo-Proxyvon (with dicycloverine and paracetamol) until quite recently. It was also used as a taper opiate for heroin detox during the 70s in the US, after "Darvon-N" (the un-injectable napsylate salt) mostly replaced the original hydrochloride. Probably not worth seeking out at this point, since it was always considerably more toxic than the other drugs in its class, and it contributed to a number of accidental deaths as well as being the main ingredient in a suicide recipe put out by right-to-die groups.
Tapentadol (aka Nucynta, Palexia) is a new addition to the category and still relatively obscure, as was its RC cousin,
o-desmethyltramadol. It is substantially more potent than tramadol and mostly lacks tramadol's serotonin effects, though it is still a noreprinephrine reuptake inhibitor, and it has been known to cause "interesting" quasi-hallucinatory subjective effects when taken in higher therapeutic dosages. This one is probably gaining ground as a global drug, both prescribed and otherwise, since it doesn't have quite the stigma of other opiate painkillers (even though the US placed it in Schedule II for political reasons when it was introudced) and has possibly slipped through cracks a bit in some countries. This is especially the case since its close relati.ve tramadol has become the number one "drug of abuse" in large swathes of Africa and the Middle East.
Pentazocine has already been mentioned, which was the famous Talwin of the "Ts and Blues" epidemic in the midwestern USA c. 1980-83. But in ampoule form (under the names "Fortwin," "Fortral," etc.) it is still a relatively common injection drug used in some West African countries and parts of Pakistan. Buprenorphine has mostly replaced pentazocine as the opiate in the "South Asian cocktail" in India and Nepal -- combining the partial-agonist opiate with ampoules of a sedating antihistamine (usually pheniramine aka Avil or promethazine) and sometimes a benzodiazepine (usually diazepam) in a large syringe for an IV injection. Which is probably a good thing, since pentazocine itself appears to have uniquely terrible effects on skin and flesh near the injection sites, causing horrific soft tissue damage, even apart from contaminants or bad hygiene.
Another member of the category is
Tilidine (aka Valoron), used mainly in Germany, Benelux, South Africa, and Bulgaria, and quite obscure outside of those countries. Like tramadol, it works primarily as a prodrug for its active metabolite, nortilidine. I don't know that much about this one, but it appears to have at least some street popularity -- I heard about it being sold alongside benzos at a certain S-bahn stop in Berlin, and it frequently comes combined with naloxone (like the new Talwin in the US) to prevent injection, which obviously indicates something.
Much older, non-synthetic members of the family include
Ethylmorphine (aka Dionine, codethyline) and
Benzylmorphine (aka Peronine). These are essentially close relatives of codeine (which was originally the brand name for
Methylmorphine) that were invented by Merck and were being marketed by around 1900. Benzylmorphine appears to be slightly weaker than codeine, ethylmorphine slightly stronger. I don't know if there are any products still available containing benzylmorphine, but ethylmorphine is still used in certain cough suppressant formulations that I know of in Francophone (Clarix, Tussethyl) and Scandinavian (Cosylan, Solvipect) countries, either by itself or combined with codeine. (It appears to be used in Turkey, Japan, and Uruguay/Paraguay as well.)