Medicine Ball/Slam ball...how heavy to buy?

Hi everybody! I'm just wondering if any of you have any recommendations on a type of medicine ball and weight to buy? I'd love to start incorporating one in my workouts...but am in unfamiliar territory shopping for one! Any advice would be appreciated! If it helps, I usually left around 12 pounds per hand for overhead presses, 8 pounds per hand for tricep exercises, 15 pounds per hand for bicep curls...