I'm Level 20 now and I need 2 Charcoal for my quest.
I looked it up on the wiki, which says it drops in a lvl50 dungeon.
Am i supposed to keep that quest until I'm lvl50 or is there another way to get it?

You basically keep the quest until you can finish it. Or you could abandon it and retake it later, etc. Either way it doesn't really matter since you can stack a lot of sidequests at one time and suffer no consequences.

It might pop up in Miroku's Bargain option from time to time. I've seen it there once, but that was back during CBT.

lol Just as I read your reply and went to check Miroku's bargain, waddya know he actually had a charcoal for sale.