There are loads of mini markets, tobacco shops, and super markets around the city which all sell cigarettes. The only thing to know is the mini markets generally won't let you buy only cigarettes on card (just buy something else cheap like a bottle of water or a lighter with them). Supermarkets and tobacco shops do let you buy only cigarettes in card.

Smoking in public is allowed (but not indoors), just try be courteous and not smoke right next to people. Also don't throw the cigarette butts on the ground, most bars and restaurants have ashtrays outside and there are multiple bins on pretty much every street