Showboat,
You are right. They are 2-inch slip joint nuts. Three out of my four toilets are antiques, have the elbow between the tank and the bowl, and use 2-inch slip joint nuts. I am able to find them (elbows too) in a small town hardware store here in Vermont. They usually come in a pack of 2 and include the washers. If your hardware stores don't carry them, a plumbing supply store will.
Good luck on your find, and more power to you for keeping your antique alive and well!
achutch