Greetings! This crow agrees heartily with your question about cigarettes! (Did you know that birds in urban areas are learning to steal cigarettes/butts to line their nest with, as the nicotine helps to reduce mites?)

There may be a few independent or black market cigarette-sellers growing it on the downlow willing to make a deal... for the right price. Feel free to create or handwave these NPCs as existing and assume a reasonable quantity of cigarettes (hand-rolled!) is available for your character to purchase, but keep in mind that they will not be overly abundant (so your poor fellow will have to be cognizant of rationing!)

Tobacco will not grow outside the leviathan due to its lack of resistance to the acid rain. However, while the topsoil is somewhat damaged and degraded as far as growing Earth-based plants would go, if you dig down about a foot, the soil is quite fertile and heavy with nitrates and can be used to grow if there is: a) a sufficient light source within the proper spectrum and b) adequate watering and space, as well as troughs or 'plots' he would need to justify using for such an endeavor. He would also need to purchase or steal tobacco seeds from an existing 'farmer' or travel to the wreck of the Adamant in the hopes of finding some seeds remaining in its vault! (Which we would help to facilitate, of course!)

If any of these options sound more interesting or engaging to you over the others, please feel free to pursue it!