Are you on the east side of Ia by chance? Presleys outdoors in bartonville IL had some great deals on ammo in their watefowl sale. http://issuu.com/kellypresley/docs/2014catalogfinal
Another thing you may consider is taking a different approach to hunting the public land. The last 2 times I hunted public marsh we out shot the rest of the blinds 10 to 1. They were all running 4-10 doz decoys, multiple spinners, and calling like madmen. We put out 15 decoys, NO SPINNER, hid real good in the grass, and just single quacked a few times to get the bird's attention. Everyone else was skybusting and chasing cripples, we were shooting them as their feet touched the water. We were only 100 yards away. The birds would flare off all the decoys, go out over the river, then turn and come right back to us.
Not saying you are doing anything wrong, IA public land can be brutal on the weekends. Just giving some experience that may help someone this season.