It is in Olivine City and in the house next to the Pokemon Center. Someone will give it to you when you talk to him.
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Pokemon Crystal only contained seven HMs. The 8th HM Rock smash isn't available until later generation games. The HM one can get in Pokemon Crystal are cut, fly, whirlpool, waterfall, strength, flash, and surf.
In Olivine City, go to the Pokemon Center. Don't go in. Then, if you're there, go left, to the building next on the left. In there, are men.talk to the man sitting at the table. He will give you the Strength HM
Outside Cianwood City Gym.
HM STRENGTH HM STRENGTH
Assuming you mean the hm fly on Pokemon crystal, you get it from the lady out side of Cindawood gym.