[pmontra]

"If you can think about it somebody already did it and it's on the internet."

Loose quote from I don't remember who, early 90s.

[YeGoblynQueenne]

You're probably thinking of (one variation of) Rule 34:

  "Rule 34: If you can imagine it, it exists as Internet porn."

https://en.wikipedia.org/wiki/Rule_34#Variations