The CircleFly Nitro cards for shotguns are easy to find. Even Ballistic Products Inc should have them in a couple of gauges. These 45cal rifle Nitro Cards are a royal bear to find though atm. Track of the Wolf should have all of the calibers and gauges in stock except these. They are listed as WAD-A on TOW. They have the over shot cards too.

Mine came from Buffalo Arms because they were the only ones with them in stock atm.