You can buy some in different pokemon centers, some are in the battle subway, you need
battle points & some people give you hms, you can always google where to find them

Hm01 cut- in striaton city. Mokomo will give it to you.

Hm02 fly- the exit of driftveil city. Bianca will give it to you after you defeat gym
leader clay and defeat her.

Hm03 surf- the entrance to twist mountain. After defeating gym leader skyla of mistralton
city, head north towards the entrance of twist mountain. Cheren will confront you and
battle you. After the battle alder will jump down and congratulate you on doing so well in
the battle. Then he will give it to you.

Hm04 strength- nimbasa city. On the second row of houses to the left, go into the house at
the very end. There will be an ace trainer (blue hair). Talk to him and he will give it to
you.

Hm05 waterfall- route 18. Instruction too obscure. Sorry for any inconvenience.

Hm06 dive- undella town. Talk to the rich girl who is standing on the edge of the "cliff".
She will give you the hm and will give you advice on how to use it.