Herba Mysticas can be obtained through five and six star tera raids, however only certain Pokémon can drop them. Even then, it's not a guarantee you'll get one. I do not know what the odds of getting one are, unfortunately.
Herba Mysticas are used in sandwiches to give you the "Sparkling Power" sandwich buff. This increases your shiny odds based on the level of the sandwich, lasting for 30 minutes just like every other sandwich buff. Sparkling Power Lvl. 1 gives you odds of about 1/2048, Lvl. 2 gives you 1/1365 odds, and Lvl. 3 gives you 1/1024 odds (All of these are without any extra boosts such as the shiny charm.).
Source for shiny odds
Everything else comes from experience. On a slightly unrelated note, here is a list of every raid that has a chance of dropping Herba Mystica and a list on how to make the most optimal shiny sandwiches.
Hope I helped!
Edit: Added in what the actual length of the sandwich buff is.
Edit 2: So turns out my first source of what Pokemon can drop herba mystica isn't entirely accurate. If anyone has a complete list, then please let me know and I will replace the source. For now, it stays however.
Edit 3: I found a better source. This link is default set to only show 6 star raids, but can be easily changed to show every other raid rarity using the drop down near the top of the page.
