Perfect Onyx: You can buy Perfect Onyx for 100 gold a piece in the Celestia Base Camp from the vendor Archytas.
Ghost Fire: You can buy Ghost Fire for 100 gold a piece in the Atheneum. Zolton Nightstone in Dragonspyre is the reagent vendor I prefer.
Blood Moss: I find that the easiest way to get Blood Moss is by transmuting the reagent. You can buy the transmute Blood Moss recipe from Avery Templeton in the Celestia Base Camp for 400 gold. It requires 15 Red Mandrake to transmute one Blood Moss.
Bone: You can get Bone from Elik Silverfist in the Bazaar.
Spring: I find that the best place to get Spring is in Marleybone. It can be harvested with Scrap Iron as a rare drop.
Sunstone: You can buy Sunstone from Diego The Duelmaster in Unicorn Way for 200 Arena Tickets. You can also buy the transmute Sunstone recipe from Avery Templeton in the Celestia Base Camp. If you are lucky, you might even find Sunstone in the Bazaar.
Amber: Amber is by far the most difficult item to get on this list. One of the most popular places to get Amber is by farming Gladiator Dimachaerus in Mount Olympus in Aquila.
Nightshade: In my opinion, the easiest way to get Nightshade is to buy it from Diego The Duelmaster for 175 Arena Tickets. You can find Diego in Unicorn Way. You can also buy the transmute recipe from Avery Templeton in the Celestia Base Camp for 200 gold.
Turquoise: You can buy Turquoise from Elik Silverfist in the Bazaar in Olde Town.
Deer Knight and Lord Of Night Treasure Cards: The best place to get both of these treasure cards is from Elik Silverfist from the Bazaar in Olde Town.
Thanks for crafting your Death spells with Ravenwood Academy! What was the hardest reagent for you to find? Which was the easiest? Let us know in the comments below.