One is from the elder sage when you beat him at sprout tower or you could buy it from Goldenrod department store for 2000PD, if not, go to Celadon and buy it there(Celadon is in the Kanto region).
5F.
you get it from a gy
Tm 70:)
sprout tower
the department store in goldenrod
The elder gives you the TM Flash after you defeat him.
flash isn't a HM but a TM and you can purchase it at goldenrod city super store thing
You can't make a cave disapear. You can light up caves with flash, and that's about it.
You need to go to the pokemart in heart gold. Go to the TM floor and you will be able to bye the move flash.
go to the veilstone city shopping mall on the 3 rd floor and buy flash from the woman
Its TM70 and you get it from Elder Li in Sprout Tower(:
Any Pokemon that can learn a electric move