You might have to look at regular magnum shotshells like these. I am sure there are others, but you may be stuck buying boxes of 25. Of course you will need to be aware of the limitations of basically a 12 ga. field load. I would limit shots to no further than 25 yards or so depending on how the gun patterns. Good luck!
I've got a 20 gauge Browning auto-5 with a 28" full choke barrel that my boys have used on turkeys. It's very effective up to 30 yds with 2 3/4" Remington magnum high velocity number 6s.