You could capture Mewtwo at the end of the game.
As for Mew, I think there was one Nintendo fest in Canada or something. So I got it. There was no other ways than getting it by hacks or through Nintendo promotional tours.
Mewtwo on the left, Mew on the right.
I never followed the show or collected the cards. Only the games. They were quite entertaining on my Gameboy Color. To think that back in the days there were such things as GameBoys with no colors! Now it's been replaced by the DS. Ah the good old days.