You don’t have to go searching for a ner for a Havdalla candle. If you don’t have one, you can still make Havdalla al hakos. If you get a ner while it is still night of Motzo'ay Shabbos, you will say Boray Meoray Hoaysh separately. (ShA 1 – MB 3)
If you do not expect to have a kos for Havdalla, still bless separately on a ner and on b’somim. If you think you might get a kos, wait and say all the berachos on a kos. (Rama 1 – MB 4)
The best way is to say the beracha on a torch of wax. (ShA2 – MB 5)
A ner of two separate wicks that join into a single flame counts as a torch. You can also attach the flames of two nairos together when saying the beracha. (Rama 2 – MB 8)
A ner with many wicks together is just a thick wick and does not count as a torch. (MB 8)
The minhag is to bend your right hand fingers over your thumb and to see the light shining on your nails and palm at the same time. If you can discern between your skin and your nails, you have enough light for the beracha. Furthermore, your nails grow as a siman beracha and the lines of your palm also contain a beracha. Some have a minhag to then straighten their fingers and look at the back of them with their fingernails. (ShA 3 – MB 9, 10, 11)
How close must you be to the ner? Close enough to be able to benefit from the light, even if you are not in the house. (ShA 4 – MB 12)
A ner that did melacha (i.e. was lit) on Shabbos is disqualified for Havdalla. If it was lit by a goy for a sick Jew or even by a Jew for a person in danger, the ner is okay for Havdalla. A goy’s own flame is assumed to have been lit on Shabbos and is therefore disqualified. (ShA 5 – MB 16, 17, 18)
To transfer a flame, after Shabbos, from a gentile’s flame is okay as long as it is not an idolatry flame. (MB 19, 21)
Light from a fire lit on Motzo'ay Shabbos is acceptable for a beracha, but not on Motzo'ay Yom Kippur. (ShA 8)
Do not say a beracha on a flame lit for a purpose other than for light. Examples: For heat. To honor the Shechina. To honor an important person. Yortzeit lamp. To honor a deadman. To honor a chassan. (MB 27, 29, 30, 33)
A blind person says Havdalla but skips Boray M’oray Hoaysh. (ShA 13 – MB 34)
If for some reason you already made a beracha on the fire, you can still say it again to be motzi your family. (MB 36)
You can say the beracha only if you see the actual revealed flame. Even if it is encased in a glass lantern, be machmir. (ShA 15 – MB 37)