This is a list of all items available for forging with Mithril, Orihalcon, and Dark Matter. I also include a list of "unique" items and all known sources of those items, such as rare enemy drops and any other item that comes with bonuses present (and isn't randomized, unlike shop/unidentified equipment)
Sword of Lord
Axe of Soul
Robe of Soul
-Knight Sword: Dropped by the "centaur knight" enemies Dark Tower (advance mode).
-Paste Sword: Dropped by any "paste" enemy in advance mode.
-Stone Axe: Dropped by "stone statue" enemies on a couple of floors of Glaybull Temple (either mode).
-Master Bow: Dropped by "archer" enemies in Dark Tower (either mode).
-Giant's Ironball: Dropped by "minotaur with iron ball" enemies in Dark Tower and Dark Castle (both advance mode).
-Shinobi Cap: Given by Kagemaru the ninja after completing Area 5 in normal mode (if all previous items are obtained).
-Bone Helm: Dropped by any "skeleton" enemy (either mode).
-Dragon Helm: Dropped by the dragon at the end of Dark Castle (not the actual boss), normal or advanced mode. Might be dropped by other dragons, too.
-Shinobi Outfit: Given by Kagemaru the ninja after completing Area 6 in normal mode (if all previous items are obtained).
-Judo Suit: Start a new Dragonute character named "Segata" and it will be equipped.
-Leaf Briefs: Start a new Archer character named "NomuNomu" and it will be equipped.
-Arrow Armor: Dropped by the blue "arrow" guy on floor 8 of Otherworldly Forest (advance mode).
-Purple Robe: Dropped by the purple "worm" enemy on floor 8 of Otherworldly Forest (normal mode).
-Royal Gown: Dropped by "sorcerer" enemies in Dark Tower and Dark Castle (advance mode).
-Bone Mail: Dropped by any "skeleton" enemy (either mode).
-Dragon Armor: Dropped by the dragon at the end of Dark Castle (not the actual boss), normal or advanced mode. Might be dropped by other dragons, too.
-Shinobi Bracelet: Given by Kagemaru the ninja after completing Area 7 in normal mode (if all previous items are obtained).
-Darkside Orb: Dropped by "demon" enemies (winged humanoids that cast magic), usually in Dark Tower and Dark Castle (either mode).
-Dragon Orb: Dropped by dragons on normal or advanced (definitely final dragon of Dark Castle, maybe others, too).
Other accessories that aren't forged can be found randomly from boss chests or random enemy drops. Some won't be found until later in normal mode (Bronze Medal is available first, for example, followed by Silver, Gold, and then Platinum). All accessories available as such can be found at any time in advanced mode (from boss chests or random enemy drops). Similarly, advance mode enemies can drop mithril, dark matter, and orihalcon, all very rarely. Boss chests can rarely have them, too. Mithril is the most common by far, due to Monuments of Mithril.
UNKNOWN ITEMS (I don't know where they are found. If anyone does find them, post about it please! Even if it's years later):
Guide by: LancetJades (of GameFAQs and elsewhere)