They're supposedly found in a grass area just north of the Pokemon centre near rock tunnel's entrance (cerulean city end).
However, to catch one you'll need a very fats Pokemon with a sleep technique or an ultra ball.
You can also catch one on route 24 and 25 (to the sea cottage though you wont get there until very late, after you beat the elite four i think)
I found it less hassle to get one from Rocket's Game Corner in Celadon City.
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If you have an Abra, the best hypnosis Pokemon is alakazam. So catch an Abra, evole it to a kadabra, the evolve it to an alakazam to have the best hypnosis Pokemon
You can't catch it in Black, but you can catch it in White. In the White Forest.
any grass around goldenrod
saffrom city grass areas
You can get a twistedspoon from a wild Abra. You can use the move Thief or Covet on a wild Abra to get a twistedspoon, or you can catch the Abra, but you might have to catch or use Thief or Covet on a whole bunch of them because it is only a 5% chance that a wild Abra will hold a twistedspoon.