Most of the Reagents can be found through normal exploration, but there is one that involves some grinding and only appears on certain nights, and that is called Wundagore Everbloom.
This resource only appears every three nights, when there is a Waxing Moon. If you hit the
button during night time, you can see on the right corner of the screen what type of night it currently is. If it says anything other than Waxing, it will not show up. So now that we know when it appears, we need to know where to get it. It can appear in ten different places, and I've found a great video that shows you every location for every Reagent, just fast forward to the last part for Wundagore locations. Please note that it doesn't always appear in every single location, it's pure RNG, so you will have to check every place.