if by "missing" you mean not in the pokedex as having a home, you wont be able to get them (zapdos mewtwo etc.. ) but you can trade them from blue red yellow gold and crystal to finish the pokedex
In the cerulean gym swim in the circle in the middle and u will find it just keep pressing a
you buy it .
he gives you the missing machine part for the generator at the power plant :) hope i helped :D
At the Power Plant.
he's on the gold bridge. if not then check the gym
Find the missing part in Cerulean City's gym. It is in the middle of the gym in the water.
the director will give u after giving the missing generator part to the manager . he will be in the radio tower of levender town
You find it in M.T Silver Cave
you will herer the police talk to a guy.he will say that a team rocket dude was spotted hanging around in mistys gym.he hides the in water floats.press a to get them.
South of Rock Cave.
You talk to the boss and he will say that somebody stole a part from the generator, when you return the part, he will give you the TM Zap Cannon. You can also now take the train from Saffron to Goldenrod.
go to pallet town, i think
surf under mauville city. you have to go under the cycle path. You then enter a cave with the generator at the end of the cave