I finally found a guy name Humphrey in Gridania next to bell in the Stalls that sell 1-5 Grades of dark matter. Do anyone know the other Sellers in the game? I thought as a big part of game NPCs would be talking about but I haven't found anyone mention it yet.
So far, all the camps nearest towns also sell all 5 grades of Dark Matter, ergo Bent Branch, Black Brush, Bearded Rock and Camp Dragonhead.
I saw a couple of quests rewarding a whole 1 dark Matter, I got something like 4 Grade 3 doing botany near camp Tranquil (Grade 3 harvesting area).
So far, I think I'm going to buy them as I need 'em instead of trying to stockpile from gathering, it doesn't drop all that often, and the price is very fair. (Better value than the repair NPC, and fixes item to 100% instead, win:win) Edited, Jul 22nd 2011 9:29pm by Docent42