It’s not about where to find them. It is how to catch them, as they seem to be impossible!
Tips to Catch Rare Fish
There are seafood dishes that increase your fishing skill temporarily, also a spinner that slows down the timer when they are off your hook (edit: trap bobber).
The higher your fishing skill, the bigger the green bar gets, which increases your chances of a successful catch.
Go to the mountain lake and get your fishing to like 3, then get some crab pots and use those to raise your fishing to 10 (or just fish normally to get it to ten but that can take a while).
Get, the iridium rod, some trap bobblers and sure bait.
Eat food that raises your fishing +3 so it increases your fishing bar even further than level 10.
Now it should be possible to catch rare/legendary fish like 80% of the time they get on the hook.
Actually you can get fishing XP with fish ponds can’t you? so yeah fishing ponds.