To elaborate on ditto if you mess it the first time it becomes available after the 9th badge I believe. You need to find day care couple for it to show in the first place and do the puzzle. I recommend saving before catching it you only get 1
Grab a pokemon you don't want and put in your party
Next in your menu go to pokegear
from there online play
from there if you don't have onle already register then log in or if you have one just Login
Then You will be requesting so head to trades then request trade
I will be waiting for the request my name is Snorlax when your ready will leave it on waiting for you
I can give you them or the pokemon accoisated with them. They should be becoming available again next episode though
PS if you want you could also PM/DM me your save and I can add them in
I can trade you both but Corsola is obtainable through your game in the dive spots in Azurine lake you can check out the first link in the spoiler in my signiture for more info on that if you want
Feel free to PM or contact me through discord