Cha_Fey_S

You can buy beast balls from the pokebay, or by doing the sidequests (alolan region)
Master balls can also be bought in pokebay, and can be earned in several sidequests.
Hope this helps.

here, you can go to wiki to find more info
https://wiki.pokemon-vortex.com/wiki/Route_16
You can find Tapu koko in route 16 in day time.

If this is the drop in sidequests, then this is when you have done the battles you need for each region and you'll get a prize (called also drop)
example: you've finished 155 battles in kanto, then you'll get a prize reffered to as 'drop'

do u mean battle?
There are 155 battles in the kanto region sidequests

It is said in wiki that ancient pokemons arent available anymore now, since they are replaced with metallic pokemons
Hope this information helps you