Go buy a half pound to a pound of frozen shrimp, being a CAG, wait for a good sale from the local supermarket. Buy the smaller size (they are measured by quantity that can fit in a pound, buy the 20 count or greater) but not the absolute smallest - each one should be the size of an Oreo or so.
If you buy the jumbo size or colossal etc., they are way too big and cost too much. Buy too small, and it's a huge hassle handling them for this special mission. Buy the uncooked ones, since these are cheaper, if you're buying them directly from the butcher/seafood area. Otherwise, buy the cheapest frozen ones that are a decent, medium size.
Now, keep a handful of frozen shrimp in a plastic ziplock bag, put it in your pocket or hold a couple of shrimp in your hand, and when you go into the store, just browse through the various sections. Find some games that no one would ever bother picking up, new or used/console sections it doesn't matter, but make sure no one is looking. Then, slip a shrimp or two in the game case (you may need to squish the DS cases shut), close it back up, and continue browsing. Do this until all your shrimp are depleted. Inside of a week, that Gamestop is going to be smelling so ripe it's not even funny, and it won't be very easy going through hundreds of cases to find the source.