Let me try to word this a bit differently from my fellow BSN posters...
You have the main game, which ends with the defeat of Corypheus. But you can still play the game afterward, run around closing rifts, complete (some) unfinished quests, etc.
Solas leaves, so he is not available.
You did not recruit Dorian (or kicked him out), so he is not available.
Even though Vivienne became Divine, you should be able to talk to her and get her to join your party if you want to continue to keep using a mage. If you don't do that, then you will not have a mage for any post-game play.
If you did not recruit a certain follower, or kicked them out at some point, then they are just gone. They don't come back at some later time so you can recruit them again. You can't "find" them again. The only exception to this is Dorian in the Trespasser DLC because that is a special circumstance.
If you want to play Descent or Jaws of Hakkon DLC and are worried you won't have a mage, you can play these two DLC before you start the final mission to fight Corypheus. You will have to load an older save for this; hopefully you have one from shortly before you went on the mission.
The Trespasser DLC is the final DLC and is an epilogue to the game. Once you start it you can NOT got back to Skyhold. When you finish this DLC, you are taken back to the main menu of the game. You can load older saves, or start a new game, but that's it.
In the Trespasser DLC, Dorian comes back, whether you recruited him or not, so you can use him as your mage. Vivienne, although Divine, should also be available as a mage. Dorian comes back for Trespasser ONLY. Nothing you do in Trespasser has any effect on the rest of the game, so Dorian stays gone in any of your pre-Trespasser saves.