Radahn can be found in the south of Caelid, more precise at the coast.
I not sure if there a other ways to get there, but I did the following (BIG spoilers ahead):
If there a other ways to get to Radahn let me know in the comments so I can edit my post.
Edit: According to Chronic Killer it seems to be possible to skip this quest line if enough advancement in the story is done. Might be worth it to skip Radahn and come back after some time if you are not interested in the quest line.