The Mew glitch is a glitch found in all Generation I main series Pokémon games and most hacks thereof, including Pokémon Brown.
Several long-range trainers exist in Pokémon Brown, but most are embedded in walls and therefore require the walk-through-walls glitch. The female Jr. Trainer in the Silk Tunnel and the Cue Ball on Route 49 are required to be fought. Therefore, the easiest LRT to use is the Youngster on Route 62.
Not all Pokémon in Brown are obtainable with this glitch, as any value from 200 to 255 will instead produce a trainer.