I was told I needed to add agricultural lime to my newly built pond. What's the best lime to use? How do I know if it's ag lime? Does tractor supply sell it?

It should be labeled ag lime. You may need multiple tons of it, depending on the size of your pond.

Check with a local feed store for buying bulk lime

Or the local grain elevator if you are in grain producing country. Talk to your USDA Agricultural Extension Office.