Players have flooded streets, parks and buildings -sometimes at night - to search for and capture Pokemon, or collect Pokeballs, which can be thrown to trap the creatures.
Called the Pokemon Aimer, the case covers part of the screen, leaving a long vertical slit in the middle which acts a guide for throwing Pokeballs and capturing monsters.
The design stops users' fingers slipping sideways, leaving them frustrated as they are unable to add to their collection of virtual miniature monsters, the Daily Mail reported.
Cleaver is selling the Pokemon Aimer case for £4.90 (NZ $9.10), but the template is free online, allowing fans to print their own.
However, a New Zealand seller has also emerged on Trade Me this week.
The Queenstown-based seller is marketing the cases, designed for iPhone 6 smartphones, on the auction site. The cases have a buy now price of $15.