How to get unique Dwellers? only from lunchbox's?

Yes, that is the only way for now. They are not hard to get though from challenges, so far i got 2 epic and 3 rare characters in 3 days without paying for lunchboxes, which is a fair amount.

Only from Lunch boxes, and the drop rate is as bad as getting legendaries in Hearthstone. But honestly who cares about special characters, not like you can show them off in this game anways. You're essentially playing this game by yourself.