Assuming that one paid relatively well, for a game like Binding of Isaac Rebirth, how much (ballpark) could one expect to pay for high-quality custom game art?

And, what is a good way to find an artist? (Forums, portfolio sites, etc...)

The price is going to vary from artist to artist as well as being dependent on the type of art that you are looking for. Some good places to check would be the job offers forum on Deviantart or Gamedev.net People will post prices and what they want to be paid for doing contract or concept work. I would check those out to give yourself an idea of what you are looking at.