I've have played the original versions for gameboy (red, blue, and yellow) many many many times. If you are looking for mew, she is only on the green version sold only in Japan. I know, I tried every trick to get mew in red blue or yellow, none of which works. (curses! foiled again!) Anyway, if you do have the green version, you get mew by finishing the whole game, (all the way thru the pokemon championship). Let the credits roll thru, then when you restart at your home, go to the second city, where the boat is, walk in the area where the boat is, do not board the boat, walk back out but don't walk by the guard on the dock, just far enough to switch screens. Watch the boat leave without you on it. Walk back out to the end of the dock. (Where the boat was.) You will be in a room that has a walkway around the edge, and a pickup truck by the door. Walk behind the truck and push it aside. There will be a pokeball. Touch it and you get a battle with mew. Use a master ball to capture her. She escapes from anything else. You need MewTwo to battle her as she can instantly K.O. any other pokemon, regardless of level, resistance, or type. Again, this only works in the Japanese green version. In the American version, you can still go to the same area, but when you move the truck, you only get a regular potion. (Disappointing.) I have also heard rumor that she is hidden in the black Team Rocket version, but have no ideal how to get her in that one.
Also, I seem to remember some trick you could do to get up to 99 of any item, but can't recall how its done. If I think of it, I will post it.