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 .
I believe it is southwest of Lavender town
In Generation I (Red, Blue and Yellow), the itemfinder is obtained from Oak's aide in building at Route 11. You must have caught at least 30 different Pokemon before he will give it to you.
first of all its just flash and two you get it by going all the way through diglets cave in vermillion town and go down through two doored place and talk to professor oaks aide and he will give you flash(you need cut to get flash
If he does not give you Flash, then you just lost the battle. He always gives out Flash.
go past Diglett's cave. on the other end, there is a house where you will meet an aide of Professor Oak. if you caught 5 or more Pokemon he will give you HM05.
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!
First you need the HM which is given by one of prof oaks aides on route 2 he's in a big building but to reach the one he's in you need to cut down the tree on that route and go to the path behind it follow it upward to find the building go inside and talk to the oak aide he will ask if you have recorded 10 Pokemon in your pokedex say YES to get HM05 which contains Flash.
no you can not catch evee in pokemon yellow but someone will give it to you
Find an Abra to give flash in the cave in dewford.
After you get to Vermilion city you go through to where ends up before viridian forest you go all the way down to where you find one of professor oaks aides and he will give you flash IF you have caught or owned at least 10 pokemon.
Prof. Oak's aide will give you the item if you have more than 40 Pokemon at Route 11.
Teach it to a Pokemon, then select the Pokemon you taught it to in the party and select Flash.