Depends what you're looking for.
If you just want to join a hunt-camp, put a post up here and ask around. If you're looking for guided (trophy) hunts, try a google search for Ontario Whitetail Outfitters. It also depends on whether you want to hunt with a bow or rifle/shotgun.
Check out Wolfe Island in Kingston for bow-hunting. It's all private land leased by outfitters, and there are some amazing deer on the little rock! Offhand I don't know if they offer gun-hunting.
Definitely do your research though, especially if you want a trophy hunt. Ask for refferences & PICTURES! It's one thing for an "outfitter" to say that there are 150" deer all over the property, but if a good year for them is 6 or 7 120" deer, and one 150", then you might want to keep looking. Also Be wary of outfitters who's main focus is bear or moose, but also offer whitetail hunts. The best Moose country is not the best deer country. Although their territories overlap, and you will find some massive deer in Northern Ontario, the deer are scarcer, and it is usually hard to catch up to the monsters in the big woods.
Unfortunately there are a lot of fly-by-night "guides" out there who claim to manage for trophy deer but are really just trying to sucker you. So if you want the hunt camp, not the trophy, just ask! I'm sure somebody here has room. If you want a trophy hunt, get references & pictures.
Good luck!
Cheers.
Tim