You cannot find hydro pump. The reason is because it is not a TM. Most water Pokemon can learn it when they have reach a certain level.
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There isn't a TM called hydro pump you can only get a Pokemon to use if it can learn it leveling up
Feraligatr can learn Hydro Pump at level 58 in Pokemon Crystal.
hydro isnt a TM.
Salamence can only learn Hydro Pump as an egg move in Pokemon Emerald.
Swampert does not learn Hydro Pump.