Black and White
At N's Castle, you can either summon Reshiram or Zekrom with the Light Stone or Dark Stone based on which game you are playing.
Black 2 and White 2
This is a post game story, if you go to N's Castle, he will challenge you to a battle, if the player wins, he will give the player either the Light Stone or the Dark Stone. If you bring it to Dragonspiral Tower, you can summon that Dragon, giving you a chance to catch it.
Omega Ruby and Alpha Sapphire
You can catch Reshiram or Zekrom at Fabled Cave with a level 100 Pokemon in your team base on which game you are playing. No item is needed here
Ultra Sun and Ultra Moon
If you are lucky, you can encounter Reshiram or Zekrom based on which game you are playing and making sure you are in the cliff world. No item is needed here either.
I hope I've helped!