Central Impotent America
Hmm, it depends.Whats a good source of cartel news?
Do you speak Spanish or are you willing to use a lot of Google Translate? If so, El Blog del Narco has been at it for years. Mainstream Mexican news sites (examples: 1 2 3) sometimes do a good covering of all things narco but to see the grimy/uncensored stuff you can get into local news groups on Facebook, just type "Tijuana/Jalisco/NL/Whatever place" + "noticias locales" / "sin censura" on facebook search or "Place" + "nota roja hoy" / "narco ejecuciones hoy" or something like that. They're deleted eventually but social media in general doesn't care about violence in shithole places so shooting or death videos often stay up a crazy long time.
In English, I only know of r/NarcoFootage and their Telegram group. They are redditors so they can be a bit annoying and sensationalist but the important part is that they translate info and news and are fairly active.