The professor gives it to you at a certain point
1 Masterball, unless you recieve a traded pokemon which is holding a Masterball
you cant
you use a masterball
nope [Edit] You get a masterball from Professor Juniper after you finish all 8 Gyms.
Team Magma Hideout
You can only get one
In team aqua's hideout
You find it in the team aqua hideout.
professor elm gives it to you..
from your enemy teams base
u buy it for 1000000
you can only find 1 masterball in any Pokemon game