Where you can find flash hm in Pokemon LeafGreen?

Answer 1

HM Location of flash route 2

when you 1st enter route 2 stay close to the right hand side until you see a tree go to the tree and cut it then travel north until you see a building entrence enter that. inside is a person with a lab coat and brown hair talk to him and if you have collected data on more than 10 different Pokemon he'll give hm-flash to you

hope it helps
The HM which contains Flash has been intrusted to one of PROF OAKS many aides he is set on Route 2 inside a building that you can only reach if you have the HM that contains Cut and the 2nd badge. There are trees placed on Route 2 you will need to Cut them down then follow the path behind the tree to reach the building where the oak aide is but now the aide will only give you the HM if you have recorded 10 different pokemon species in your pokedex if you have done this say YES to his question to get HM05 which contains Flash.
Go through Diglett's Cave (East of Vermilion City), then when you come out, head south. Inside the big building (not the house) there is one of Professor Oak's Aides. Talk to him and if you have caught 10 pokémon, he will give you the HM for Flash.
you defeat the electric gym leader and he gives it to you

Answer 2

Once you get the HM Cut go to viridian city and go north cut down the tree on the right and head north you will enter a big building in there will be a oak aide who will ask you if you have 10 Pokemon recorded in your pokedex if you say yes he will give you HM5 Flash.

Answer 3

You gotta go to Deford then go up then go right and you will see a cave go in it. After you go in it you will see a person talk to him he will give you flash.