Hi Serephis,
I'm not sure farming reagents is the easiest way. The way I did it, was earn some gold, then buy stacks of reagents from felu vendors for 2-3g each. Not sure if you know, but when the vendors refresh (once an hour or two) if a stack of reagents has been bought out, they will 'double' it until they reach the cap of 999. In felu, reagents are cheaper and you can get 999 of each for about 20k total. If you have time to occasionally visit 4-5 vendors in felu over the span of a day, you'll have 50-60k reagents in no time.
To answer your original questions, liches, fire steeds (300-500 sulph ash), and mages all of the world drop a mix of different reagents you can farm if you still wish to do that. You can find liches near yew, stygian abyss, and many other place. There is a link to fire steeds in the NEW library, and mages are in any dungeon, including wisp in ilshenar and third level of shame.
Alexandra.