Things that are impossible to get in Montenegro?
Plastic zip-lock bags.
A Starlink subscription. The Powers That Be want the ability to pull the plug on all Internet service should it suit their political needs. (Note: most Internet service passes through routers in Serbia, so Serbia can also pull the plug.) The government has created very high barriers to offering Starlink service subscriptions.
Clean breathable air in Podgorica in November through February. The area has the worst air in Europe in the winter, mostly because there is no natural gas (methane) infrastructure and wood burning for heat and cooking is widespread. They are also burning cheap high-sulfur ignite coal for power. The bad air is estimated to take 1 to 2 years off your life expectancy (and make you sick long before you die).
And yes, as answered above: Nicorette gum (or any other brand of the same). If you try to import it, you'll have serious issues with Customs. It may be easier to import weapons-grade uranium ;-) The tax authorities want you to continue paying tobacco taxes, and the pension funds want you to die sooner. Combine tobacco smoking with the bad air and the pension funds have a much lower payout.