Where do you catch a beldum in Pokemon liquid crystal?

You can't catch Beldum in Pokémon Liquid Crystal. Beldum is a Pokémon introduced in Generation III (Ruby, Sapphire, and Emerald), while Pokémon Liquid Crystal is a fan-made game based on Generation II (Gold, Silver, and Crystal).

Therefore, Beldum doesn't exist in the Pokémon Liquid Crystal world.