PPSSPP from the Play Store, ISOs from trusted archive sites (Reddit’s Roms subreddit is your friend), and mods from GBAtemp or Nexus Mods.
The PSP did have two fantastic Call of Duty games: Roads to Victory (WWII) and a surprisingly decent port of Call of Duty 4: Modern Warfare (often mislabeled as Black Ops 3 by modders). Clever fans have reskinned these games. You’ll see Black Ops 3 menus, futuristic HUDs, and custom Zombies maps slapped onto the Modern Warfare engine. call of duty black ops 3 ppsspp download for android