Hello: The most common species to stock ponds are sunfish species, largemouth bass and also catfish. It is not recommended to stock crappie in small ponds. Bass are needed to feed on the sunfish species to help keep the fish population in balance. Stocking catfish is an option, and generally the numbers are low, 2-300 per acre. The following link provides a good overview of stocking ponds and managing ponds for fishing: https://extension.psu.edu/pond-fisheries-management. As far as suppliers, first start with your state fisheries management agency. They may offer stocking or should have a list of local private suppliers. That office will also likely have information on managing fish ponds in your state.
Good luck with your fish pond!