Flash is given by an oak aide on route 2 to reach the oak aide you need to use Cut on the small trees then go inside a big building inside talk to the oak aide and he asks you if you recorded 10 species of Pokemon in your pokedex if so say YES he gives you HM05 which contains flash.
http://wiki.answers.com/Where_is_flash_in_pokemon_leafgreen#ixzz17L2EK8NL
on Pokemon yellow on Pokemon yellow
Flash can be found on Route 2 in Pokemon Yellow Version.
You can find it either before you go into the forest at the start , or you can find it when you use cuts if nearby the digletts cave .
pikachu or electric Pokemon ......
Cut, Fly, Surf, Strength, Flash.
catch ten or more pokemon. find oaks aid near viridian city and talk to him. he will look at your pokedex and if you have ten or more pokemon, he will give you flas. hope this is helpful!
To navigate through Rock Tunnel
Whirlpool isn't in Pokemon Yellow Version.
The place where it flashes is where you can find the Pokemon It would flash blue if it was found in the water
Do the same thing in Pokemon peaRL
catch 10 different Pokemon go through diglet cave then go down use cut go into this place and talk oak's aide and he gives you it
Pokemon Yellow does not have meowth and it can only be obtained by trade.