In the game - unfortunately they are nowhere to be found. Once you finish the main storyline, you'll notice a box that comes up and says something to the tune of ...
"Geralt has reached the end of his story. You can continue to explore the world and complete contracts/quests. This places you in a timeline shortly before the main battle."
All of your inventory remains in tact. You can continue to complete quests/contracts and explore. The main story is just ... over. You can find Dandelion at his inn. Zoltan seems to be missing. Keira can be found at Kaer Morhen selling merchant items, but that's believed to be a bug, since you can only reach her by scaling the castle wall and it's difficult to get back out of the area without dying. I haven't been able to locate any of the other characters.