An .nkit file is a compressed version of a standard Wii or GameCube disc image (.iso).
The "Megathread" in this subreddit is the gold standard for finding safe links to almost any game library. Quick Tips for .nkit Files: Download WWE '12 (USA) (En,Fr,Es).nkit
Are you planning to run this on or through an emulator ? Download WWE '12 (USA) (En,Fr,Es).nkit
This is often the safest and most "official" way to find preserved game files. Search their "Vimm's Lair" or "Redump" collections. Download WWE '12 (USA) (En,Fr,Es).nkit