You cannot buy a DSi Action Replay at GameStop.
Action Replay is a third-party, unlicensed accessory. GameStop's policy is to sell only officially licensed products (by Nintendo, Sony, Microsoft, etc.). Unlicensed devices like Action Replay, GameShark, or generic flash carts are not stocked.
You may find them on Amazon, eBay, or specialty retro shops—but be aware:
Stick to official accessories for safety.