Go to Saffron City and enter the gym and find a way to get to Sabrina
You get the Marsh badge from defeating Sabrina, who is the gym leader in Saffron City. Sabrina uses psychic Pokemon.
Go to the forth gym!
You will have to defeat Bugsy in Azalea Town. He has bug-type pokemon.
The 8 badges for Pokemon Red are: Boulder Badge, (Brock) Cascade Badge, (Misty) Thunder Badge, (Lt. Surge) Rainbow Badge, (Erika) Marsh Badge, (Sabrina) Soul Badge, (Koga) Volcano Badge, (Blaine) and Earth Badge. (Giovanni)
blackthorn city
it is bugsy the bug type leader
In Mahogany town right from Etrack city
It is your rival from Pokemon fire red and leaf green versions.
you have to get the cianwood gym badge and the olivine gym badge
The Marsh Badge is held by Sabrina the leader of the Saffron City gym.
you get it in ilex forest after you get your 2 badge! hope this helps
in Goldenrod city