If you don't have prizes on-hand, please don't start a competition.
It appears you're looking to commission someone, instead. There are a few places you can look in the Oekaki, or you can try the
Art Shops board.
That may be the better route, really, as Oekaki competitions require a lineart of full background (a setting, like a field or forest). This isn't really possible in this case. ^^;