From the elder in the sprout-tower, after beating him of course.
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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.
sprout tower in violet city. you have to win against all the monks. geeeze
All electric and normal. Not sure of others, but I think almost all can. Not all normal Rattata can't learn Flash
Go to the top of Sprout Tower in Violet City and battle the guy who your enemy guy just battled. When you beat him he will give you HM Flash.