you can also get it on route 111, theres a guy standing in front of a big tree who gives it to you.
In slateport if you go to the lower left of town at the market place before the beach you can buy secret power from a lady there... it is TM 43. -hyak
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In Slateport City (after you get the TM from the guy on that route), in the market place, a merchant will sell you the TM Secret Power and the TM Hidden Power.
If you go through the dessert in Pokemon emerald, Sapphire, or ruby, you'll see a boy standing next to a tree. Talk to him and he'll give you the TM Secret Power.
it is TM number 43
Get the TM secret power teach it to a Pokemon then find either a tree with vines, an indent in the wall or a big bush use secret power on these to create your own secret base.
NOWHERE.The TM trick is not available as a TM in Pokemon Emerald