Others get them buy buying them. Usually it's a process of a couple of hours to get the vendors to sell thousands at a time, since they double every 20 minutes to half an hour (I'm not sure exactly) up to at least 640. I haven't gotten past that in my reagent buying outings yet.
Or you can pay the extra gold, save the time and buy 'em from player vendors.
My necromancer only gets reagents by killing lich lords and devourer of souls.
I suppose if you killed mage lords or juka mages for an eternity and a half, you could come up with a respectable amount of reagents, but it's so much easier and efficient just to buy them.