I've been to both Dragonspiral Tower and Giant Chasm, but I haven't found Reshiram or Kyurem. I've already beaten the elite four and Iris. Why can't I find them?
Go to Victory Road and look for a a Zoroark NPC. Follow it to reach N's castle then defeat Reshiram. Head to Dragonspiral Tower and catch Reshiram and after that head to the Giant Chasm(where you defeated Ghetsis) and catch Kyurem