You get one from Cyrus, after beating him. Then...
If you still have your Masterball from beating Cyrus the first time then you can try to clone it using the GTS cloning glitch which also works for any Pokemon and item. It involves giving a Masterball to your a Pokemon and then offering it as a trade, but this needs to be done quick and make sure that the deal you choose will be one that no one will trade for. You have to find out how long it takes for your game to upload the pokemon to the GTS by counting the clock in the bottom right corner by 1/8 increments because what you need to do is shut your DS off 1 clock tick before your Pokemon is uploaded. On average it takes around 8 clock spins, but the amount of time can change it take can change every time so try to keep your DS in the exact same spot when doing this such as by setting it down onto a table. When you turn your DS back on it should say your save file is corrupted, but don't worry, it will load the previous save file (which will be when you were forced to save to first access the GTS) and if all goes well, you should have a Pokemon in your party or PC with the Masterball as well as an exact copy of this Pokemon with another Masterball up for trade in the GTS. Take it back before someone trades for it!
If you already used your Masterball from Cyrus then you can try to win the daily lottery in Jubilife city. You can't just match a few digits, the lotto ticket number you draw has to be an exact match of an ID number of one of your Pokemon if you want to win a Masterball. The pokemon can be in your party or your PC, but to increase your chances of winning, trade with other people on the GTS or with your friends since traded Pokemon you receive will have the ID number of the original trainer it was with.
One more way is if you can find a friend or trade offer on the GTS of a Pokemon that is holding a Masterball and you trade for it.
Any of these methods can be used to get more Masterballs by doing the GTS cloning glitch, and just so you know, the news guy in Solaceon town does not give out Masterballs, in less this rumor below is true: