You can take your failed breaker, and whatever information is on the box from labels, logos, etc. with you when you go to the store. Knowing only the probable age of the box, you're unlikely to find the breaker at the local Home Depot. Even if they do have it, it is unlikely that you will find a store employee who can authoritatively tell you that the breaker will work with your box. In that situation, you going to have to go to an electrical supply store or hire an electrician. Now it may turn out that the box was replaced subsequently, or it is a GE or something that is still around, and you find a drop-in replacement for it. So go ahead and look for it. Just don't pop it in until you're sure it is made for the box.