Starters are very hard to find in the wild. Before this update, they had the same rarity as regular Pokemon.
However, now finding a starter in the wild is a rare event.
Vortex Balls are the same as Master Balls i.e they have a 100% catch rate. The difference being you can even catch Ultra Beasts with them. This is not achievable with a master ball.
Vortex Balls can be won in Pokebay Auctions or you can buy them with real money from the forums store.
Hope I helped!
-Waybig