In poke marts
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to buy a ultra ball in Pokemon crystal version , you must go to blackthorn pokemart and pay 1200 for each one. but its kinda far in the game . the last gym leader lives in that town.
If you are in battle with another trainer you cannot catch that Pokemon that belongs to them. But if you find out where that Pokemon's habitat is (by using the pokedex) then you can go and catch it with a poke ball, great ball, ultra ball etc...
In Pokemon Crystal.
No, you do not need a master ball, if you get it to a low enough hp, you can catch it with an ultra ball. You can find mew in the end of the cave in cerulean.
What the hell is a Pokemon crystal