I'd say you have three options:
- Explain the situation to your neighbors, and hope they're the agreeable sort.
- Give the aforementioned neighbors an equally noisome "incentive" to smoke there no longer, perhaps getting yourself evicted or even arrested in the process.
- Negotiate with the realty office.
The reason you can't contact the landlord is because he doesn't want to deal with you (or anyone else, for that matter). That's why he's paying the realty office a large sum of money every month to do it for him. If they do give you his contact information, then they're putting their own livelihoods at risk.
As for the mentioned solutions, number one is fairly straightforward; I won't comment on number two; and number three basically comes down to threatening to move out unless the issue's resolved. Consider, however, that if the smokers do not relent, then the realty office has to make a decision as to which renters to keep. That decision will probably center around who's paying more rent.
Good luck.
Edit: Also, check the local laws on renting. There's usually something about tenant rights, and so you may be able to go to court.