There are some here on the forums in the
Guides section and on YouTube. Don't limit your search to just Mobile farms. There are lots that are made on PC and console that may work in mobile. Some won't, but many might.
A search for "Terraria key mold farm" on YouTube will get you lots of options. The ocean grinder seems to be the best way to go. And if there is a farm that you like the look of, but it uses spears traps and you don't have any, try something else that you do have (regular dart traps will work but will just take longer), along with actively killing the enemies while you're standing there (instead of using traps).
Good luck.