Rock tunnel is pitch black, where do i get flash in fire red?

you can walk rock tunnel dark too, it's annoying but doable, walls still block you and trainers still find you. if you do teach flash, put it on something you don't mind losing a slot on. hm moves stick until you see the move deleter in fuchsia.

One of Professor Oak's aides gives you HM05, in a small building on Route 2 (the bit you reach through Diglett's Cave or with Cut). He only hands it over once you've caught 10 different pokemon. You need the Boulder Badge to use it outside battle too. You can walk Rock Tunnel in the dark, it's just miserable.