How to get more master balls?

Can't, you can only get one and thats all.

If you still haven´t used your masterball yet you can give it to a Pokemon and then breed that Pokemon with a ditto and when it hatches it will be holding a masterball

Thisworks with all items either Trade it to Pokemon Emerald goto the battle towers pc on the upper left deposit the Pokemon you want to clone turn of the pc save when done saving turn of game console turn back on go to closest counter when it says do you want to save the game yes or no when the dialog box is complete turn of the game console without saving turn back on Pokemon will be in box and pc note up to 5 Pokemon can be cloned at same time you have to sacrifice one Pokemon box or party you must fix network machine on island 1 to trade from Kanto to hoene cloned Pokemon will be holding same item the other way is on island 4 put to Pokemon in daycare center if one Pokemon holding item baby will be holding item legendarys don't give babys because they don't have genders only heatran gives babys heatran found in sinnoh if you put Pokemon such as Charizard ,blastoise or venesaur with ditto found east of fuchia city poisin gym leader city as I was saying puting your starters final form will make either baby Charmander baby Squirtle or baby bubusaur at level 5