These coffins are sold in Bazzar, the vendor in the middle building, right as you go in front is first guy on left, can't remember his name. The lower lvl spell uses the bigger non stackable kind which really sucks, the higher lvl spell uses the stackable kind, so you can summon more corpses after some bad luck on a raid:).
Location is correct. His name is Dashelo Dalroshan and, unless someone sold it to him, he has all 3 coffin types for sale. Tiny Jade Inlaid Coffin for under 70 plat. Jade Inlaid Coffin under 150 plat, It weighs 10.0, holds 2 Tiny size items. Black Ceremonial Coffin for under 2 plat.
I dont know of any tradeskill these are used for. You are correct Domeroth, coffins are used by Necromancer/Shadow Knights for the summon corpse line of spells. There are 3 different coffins available. The Black Ceremonial Coffin is used for the first of the series spells, lesser Summon Corpse. The large Jade Inlaid Coffin is used for the higher level Summon Corpse, and the Tiny Inlaid Coffin I beleive is used for the higher level Conjure Corpse. Although, I am not high enough level to test the component for the Conjure Corpse spell. I hope that helps some.
I know this is used with the shadowknight/necro(?) spell, "Conjure Corpse" so where do you buy the things? I've also been told they are the only stackable coffin in the game to make corpse summoning easier.
Let's see, I have seen them sold in the Plane of Knowledge, the Bazaar, by gypsies in various places on Norrath (Lake Rathe for one), and I heard in Paineel and Neriak.