How do you get ultra rare Pokemon in Pokemon Ruby?
You catch them. Or trade for them.
carry a lot of ultra balls and just try to catch them, and if you have ruby, Sapphire, or emerald you can trade Pokemon from you gamecube from the Pokemon games like Pokemon xd gale of darkness, and Pokemon Colosseum. so you can get rare Pokemon that way, i had to spend 5 hours trying to catch rayquaza on emerald so it takes a while