Where can I find stock or custom ROMs for my Android device?

There are several reasons to be looking for a ROM: - Upgrading or downgrading your stock ROM - Looking for firmware from a different region/language - Flashing a custom ROM for different features, removing bloat, etc. But where can one find ROMs in the first place? What sites are likely to have the ROM I need? *[This question is intended to be canonical](https://android.meta.stackexchange.com/questions/782/where-can-i-find-x-rom-questions-should-be-treated-like-shopping-questions) ; please add to the wiki answer instead of posting separate answers.* See also: https://android.stackexchange.com/q/13510/1465