Take a piece of 1/4" rope and wrap it around your pulleys (less AC) but around your tensioner. This will give you a ball park of the belt length you need. It will also show if there's anything in the way of your new belt. (HINT: overlap the ends then use a marker to draw a line across the rope. Makes measuring easier and you can get a good tension on the rope.)
Now here's the hard part, finding a parts store with an employee that knows how to use the reference material they have available. Knowing the width and pitch of your original belt (from the reference material) and the length of the belt you need (from your rope) they should be able to tell you if a belt is available that fits your needs.