Hasbro (the publisher) does not host a dedicated PDF for every licensed Monopoly variant, but you can almost always find the rules using these methods:
"SpongeBob Monopoly" rules filetype:pdf or "Monopoly SpongeBob" instructions.If you know standard Monopoly, you only need the variant-specific rules: property names, special spaces (e.g., "Jellyfish Fields" = Free Parking?), custom cards, and any unique mechanics (like the "Krabby Patty" token ability in the 2019 edition).
Pro Tip: Take a photo of the rules sheet with your phone next time you play—never lose them again!