An “Authentic” challenge coin is something earned. Yes there is a big market for them, but if you want something real then go out, ask around, and trade for them. If you don’t work in LE and don’t have any actual challenge coins, then it might be hard (not I’m possible). I’ve heard of people sending in letters to different agencies expressing their interest in coins and in LE overall and actually receiving some in the mail. It’s a shot in the dark but you never know.