they have biodegradeble golf balls on cruise ships that turn into fish food.
Had a quick look on aamzon and can't find any.
Clay works for me.
I like a 60-70 gram ball.
Tennis balls also work well and are easier to find.
Though you'll never leave with the same number you arrived with
Second hand used balls are also very cheap, and will last a LOT longer than the cheap ones you get for dogs.
But if it's a mowed field then air dry clay is your best bet. Once rolled and left for a day or so you can spray paint them bright colours - we had orange flourescent ones at hodd hill.