You get it on the opposite side of the route before viridian forest. You will need cut for that, so you must have completed the SS. Anne.
a city
in front of a cave not sure which 1 tho
other side of diglett tunnel
one of professor oaks aides gives it to you
get the item flash
it's either cut or flash i cant remember
use the move Flash (TM70)
cut,fly,surf,streangh,flash,rocksmash and water fall
It is east of cerulean city, HM1 CUT is needed and HM5 Flash.
one of oaks aids has it by route two after somthing and its called flash and its an hm
First you need the first badge then you need the HM Flash which is given by the oak aide on route 2 lastly go inside rock tunnel and choose the Pokemon that has flash then choose flash the cave will be lighted up more!
Get 10 kinds of pokemon if you haven't already, then go thru diglett's cave. You'll come out near a building. There will be an aide there. Talk to him and you get flash. Then go to rock tunnel, after teaching your pokemon flash.