Regular bottle caps can be fished up, obtained from a Battle Tree streak daily, or obtained via Festival Plaza lottery. Golden are from lottery or Aphun.Welp got a 5/12/31/22/31/31 Modest Lunala in a Dusk Ball first try.
I guess once you have the nature it doesn't really matter this gen (unless you're going for a certain Hidden Power or 0 speed).
Are bottle caps post game? Any guide on the best ways to collect em?
Battle Tree streaks require 30 wins and you to lose and collect your prize the next day. If you like Battle Tree, this isn't too bad.
Aphun is just luck.
Treasure Hunt can be manipulated by buying a 2** booth, using the lottery, and replacing said booth repeatedly for bottle caps IIRC. It costs festival coins, but it's the best way to farm them quickly.