There are 26 Pokemon that learn Hypnosis in Generation III.
By level-up:
* Poliwag - level 7
* Poliwhirl * Poliwrath * Gastly * Haunter * Gengar * Drowzee * Hypno * Exeggcute * Exeggutor * Hoothoot - level 16
* Noctowl - level 16
* Politoed * Stantler - level 19
* Ralts - level 41
* Kirlia - level 47
* Gardevoir - level 51
* Spinda - level 23
* Lunatone - level 19 By breeding:
* Vulpix (father: Stantler / Spinda)
* Meowth (father: Stantler / Spinda) * Psyduck (father: Poliwag / Poliwhirl / Poliwrath / Politoed / Stantler / Spinda)
* Ponyta (father: Stantler / Spinda)
* Mr Mime (father: Drowzee / Hypno / Spinda)
* Feebas (father: Poliwag / Poliwhirl / Poliwrath / Politoed)
* Chimecho (father: Gastly / Haunter / Gengar / Ralts / Kirlia / Gardevoir)
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you can catch a ralts on route 102 and put an everstone on it, then it will learn hypnosis at level 41
Drowzee and Hypno are top choices. but most psychic Pokemon know/can learn Hypnosis.
Munna and Sigilyph.
Its not a TM. Your Pokemon has to learn it. like Gastly :) (Gengar in lvl 100 with hypnosis is pwnage :p)
Only certain Pokemon can learn it but you can't find the TM