MissingNO can be found in Yellow version! The most obvious way to obtain it is to trade it from Red/Blue to Yellow. Here is another way, which is highly dangerous, and seldom works. Here are the steps.
1. Walk up to this Gambler (this will not work if you already battled him),
but Fly away before he sees you. A ! should come up over his head if done correctly.
3. Find this Channeler and have her battle you. It helps if you do not run into any Wild Pokemon on the way back to the town.
5. Go outside and walk towards the opening to the west.
The menu should come up. Close it and you'll find a MissingNO at lv. 7. (Sorry for the blurry picture, that's the best I could get without it freezing up...) This MissingNO could wreck your game if you face it, and if you do manage to battle it, there could be side effects after the battle. One time it said, "MissingNO learned!" over and over again.
NEW DISCOVERY! New MissingNO types discovered! Click HERE to find out more!
Note: These kinds of Pokemon require you to catch them twice, as if you see its Dex entry, it will turn into a Rhydon.