you can buy them in the indigeu plateu
You can buy Ultra balls from any Pokemart after you defeat the elite 4. The cost £1200 each.
you can't buy ultra balls until you beat the 5th gym
Buy ultra balls in Fuchsia city or Indigo Plateau.
You will be able to buy Ultra Balls at all Poké Marts after earning at least three badges.
You can buy them at the indigo plateau or at Sootopolis city.
You can buy them in the Pokemart in Pastoria City
Usually ultra balls are sold in the later cities try blackthorn city or the Pokemon league.
all after where you get the fifth badge
He is at mt.silver bring lots of ultra bals and timer balls he is level50
throw a master ball or buy as many ultra balls and hope fully you'll catch him cuz i havent yet.
Use your strongest Pokemon, do not attack, buy 15 Ultra Balls, 5 Great Balls, 20 Poke Balls, and 6 Timer Balls. Keep using the balls, save 5 of the Ultra Balls then use a few Timer Balls. They are guaranteed to catch Deoxys.
first buy as many ultra balls you can and battle Mewtwo. Once his hp is low start using the ultra balls (just keep trying until you got him)