General

@KaichouShion heart stones still exists.

OT: best way, would be farming for hyssop flowers, junipter seeds/lowers, lidium ore, wisdom/luk crystals at evo world. (use 70/100% population, drop rate core, ore drop, ore drop rate, flower/seed drop, and flower/seed drop rate.)

Then, refine them, and make them into crystals and oils. When you do this, there is 10% chance of obtaining masic powder(purple).
(this "10%" isnt official far as i know, but from my long experience, i came to this conclusion for rate)

i find a bunch of magic powders from the mysterious veins and herbs. usually i just find the herb ones though. i think the gold flower and heart stones drop powders too. i don't really look for them, they just appear

For mining:
Look for the Purple Veins, Heartstone, or Legendary Veins. idk if the heartstone are still available lol i havent seen one since legendary came out
The purple will let you get Lidium, Wisdom, and Luk ores, and when yo craft the larger version, you have a chance of it being the powder.
The Heartstone/Legendary will sometimes drop a powder, or the ores of the items^

You get a chance to get purple powders from refining Juniper Berries and Flowers, Hyssop Flowers, Wisdom and Luk Crystals and Lithium.

First of all, know that magic powders are the result of crafting (only sometimes), and purple magic powders should be the result of refining purple herbs or ores. For herbs it should be Lavender, and I'm not sure about ores.
So to farm these herbs and ores for refining, you should use Evo Lab primarily. Harvesting them are much less efficient especially if you have drop rate cores. Also if you do the Part-Time Job for Mining or Herbalism, you'll be rewarded 30 of a certain herb or ore, which may be the purple one you're looking for. You also might get a recipe from the Part-Time Job.

Mine in Ardent at the Expert field with the Purple Veins. Then I guess just craft till you get enough ._.

The Free Market.