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; please add to the wiki answer instead of posting separate answers.
See also: How do I update Android on my device?