You get it after you defeat Team Plasma in Pinwheel Forest.
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you can get one from lenora when you get the dragon skull back
Zekrom (the black dragon) is found in Pokemon White.
Unfortunately you cannot obtain Reshiram (Dragon/Fire) in Pokemon White, rather you can obtain Zekrom (Dragon/Electric). In my opinion, Reshiram is better, so i got Pokemon Black.
If you are talking about the 2 mascot Pokemon then you have the following: Reshiram - The "Vast" Pokemon. Reshiram is Fire/Dragon type and is only catchable in Pokemon Black. Zekrom - The "Deep" Pokemon. Zekrom is Electric/Dragon type and is only catchable in Pokemon White.
Dragon Dance can be learned by many dragon types such as Dragonite.Dragon Dance is not a TM, and can only be found on Pokemon.Try looking up the move Dragon Dance to find Pokemon that can learn Dragon Dance.