sadly there are only 3 1/2 ways
1.- getting it from a friend who has it
2.-getting it on a official tournament made by Nintendo company. unfortunatelly those games are not the "newest" so there may not be many tournaments
3.-getting it from cheats on GameShark/action replay
3.5.-sometimes in those freak events where many freak people go (convenciones en español), you can find stands with people selling Pokemon by cheap prices (they bought a gameshark or something and made bussines)
Darkrai.
You can't without using cheats.
Not without trading from another game or using cheats.
No, not that i know of.
To get event Pokemon without using cheats or events is simply impossible without trading. If trading is not an option you must attend these events or use a cheat device.
That is impossible.
you do not..
You can get ditto in route 218 by using a pokeradar i think?
You can't without cheating. Sorry
in Pokemon yellow, blue, red, fire red, and leaf green you can find mew-two in cerulean cave after you have beten the game, but for mew at the moment you can not get it without cheats...
No, you need to go to a Pokemon Event if you want a Mew without using Cheats or Action Replays.
Trading, Migrating, Catching the legit legendary Pokemon in pearl, etc.