It depends upon what type of balls you like, I like to hit the Titleist "Pro V1", prices vary substantially for other types and makers of golf balls.
In answer to your question, nobody sells new golf balls individually. You can buy a "sleeve" of 3, or a dozen (some manufactures sell packages of 15 or 18 as well). At the course, you can buy individual balls which they fish out of the water hazards, usually for $1.00 (many are like new, if not actually new . . . they may have decided to take a swim on their first time out!).