I dont know of any site where you can buy a monetized YT channl.
That being said, if its a legit channel that has +1000 subs and several videos, that will not come cheap.
I think you will be better off, just make 10 videos of whatever (I seen videos of birds talking for 3 hours being monetized with ads), and buy subs and views from Fiverr or whatever site you prefer..
That should be able to get monetized,