All information to get the rare items is already in the game, if the Stranger talks with every character. Although the NES remake has shorter dialogues, most information is still found in the same towns and also at the same pubs.
Magic reagents
Nightshade fungus
Virgil reveals the coordinates of the nightshade fungus.
The first clue about the nightshade fungus comes from Presto, a druid in the Skara Brae apothecary; he directs you to the pub in Vesper, then the companions are directed to Virgil, a mage in Trinsic.
All the thread can be followed by moongates: start in Skara Brae, take the moongate to Minoc and visit Vesper, then the Minoc moongate to Trinsic, and you are already rather close to the coordinates of the fungus.
Three spells only require Nightshade fungus:
Jinx: useful against large numbers of enemies;
View: useful, but it might be more convenient to buy magic gems instead;
Kill: not useful, ranged weapons are more reliable.
Mandrake root
Calumny reveals the coordinates of the mandrake root.
The trail for the mandrake root picks up in the same city where the previous one ended: Trinsic. Swindrik directs the Companions to the pub in Paws, where a tip of 99 GP reveals that Calumny of Yew holds the final information.
Calumny tells you the two locations of the spots where mandrake root grows: in the plains south of Minoc and in the "Fens of the Dead" (the swampy delta south of Paws), but the latter location is easier to identify. As the Nightshade fungus, Mandrake root can only be found "on the darkest of nights".
Purchase a good stock of ginseng and garlic in Paws, and mix a good number of "Cure" spells. Even better: purchase reagents for just 8 Cure spells, overpay the apothecary, and restock after every trip to the Blood Plains, so to improve Honesty and Justice.
Seven spells require Mandrake root:
Jinx: useful against large numbers of enemies;
Negate: useful against enemies that cast debuff spells (sleep) on the Companions;
Resurrect: absolutely necessary, although hopefully used not often;
Gate travel: use sparingly, because walking allows to fight and earn experience and gold;
View: useful, but it's more convenient to buy magic gems instead;
Tremor: not useful, ranged weapons are more reliable;
Iceball: not useful, ranged weapons are more reliable.
Spell recipes
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Original spell quests
Undead
This spell is left as a riddle for the player. In order to solve the riddle, the player should read carefully the manual.
Excerpts from the manual
About undead enemies and the Repel Undead spell
These once-dead beings take many forms, the most common being the animated skeletons of orcs and goblins, or the ghoulish forms of flesh-eating corpses that have been summoned from the land of shades to wreak havoc on the living. These apparitions are cowed by the light of righteousness and fight as warriors in a trance.
You must use your knowledge of the properties of magical reagents to determine which two will lend force to your enchantment of Undead turning.
About the reagents
Garlic is used in all spells of the warding variety, be they the warding off of common and magical sickness or the repelling of beings once dead.
Sulfuroush ash is useful in the casting of Energy Fields and Magic Missiles, as well as in spells requiring a quick burst of light or a sustained glow.
Praise the wizard who realized the mystic secret of spider silk, for it is to him or her that we owe the knowledge of binding and restraining spells.
Ginseng is most useful for spells of a healing or narcotic nature, such as Cure or Sleep enchantments.
Blood moss is used in the spells of movement, from the simplest levitation to making the very earth tremble.
Black pearls are vital in the casting of spells that are hurled from the mage's person and must travel to a final destination.
Mandrake root is the most powerful known substance in the weaving of magical spells.
The chief magical properties of nightshade fungus are connected with the use of poison and the creation of illusions so real that they can lay the mightiest warrior to the ground
Therefore, in order to make an Undead spell, garlic and sulphurous ash sould be mixed.
Resurrect
Shazom, a dying young wizard (in Moonglow)
JOB? I am apprentice to the great wizard Nigel!
HEALTH? I will soon die.
DIE? Pass on.
NIGEL? I will soon need to use the spell he calls 'recall'. Hast thou met Nigel?
NO? He lives at the Lycaeum.
YES? Seek him out for he will teach thee.
N/A
Nigel, a noble wizard (in the Lycaeum)
JOB? I teach magical spells.
SPELL? I have a specialty! Dost thou know what it is?
YES? What do I call the spell?
RECALL? Yes, resurrection! It takes ash, ginseng, garlic, silk, blood moss, and mandrake!
A bearded mage (in the Lycaeum):
I am Nigel, master of all wizards! My magic is strong enough to raise the dead! Life, thou knowest, is given only unto the highest of wizards, and it requires garlic, moss, fungus, and... hmph, well... and... and Marina of Empath Abbey will tell thee its final ingredient! I am quite busy right now, no time for chitchat!
N/A
A beardless mage (in Empath Abbey):
Hello! I am Marina Mandracha, in the service of Lady Marcy.
Gate travel
Jingles, a young mage (in Paws)
JOB? I seek the wisdom of magic.
MAGIC? My master knows the gate travel spell!
MASTER? My master is Mentorian. Dost thou know him?
NO? He lives in a hidden village in Lock Lake reachable only by ship, ask of the gate spell!
N/A
Mentorian, a tall wizard (in Cove)
JOB? I study the magical arts.
ARTS? There is truth in magic! Dost thou disagree?
NO? Good.
GATE? Since thou dost bear the ankh, I shall tell thee. A gate spell requires ash, pearl, and mandrake root!
A bearded mage (in Cove):
Ah, seeker, we meet at last. I am Mentarion, master of the Gates! Know now that sulfur ash, black pearl, and manroot are necessary for the Gate spell.
If, at this point of the game, you still need thousands of Gold Coins to purchase the best equipment, you might decide to stop before exploring the dungeons. In fact, most of the gold collected in dungeons will have to be used for resting/healing and to restock on magic reagents.
Similar to Ultima 3, the top floor of one dungeon holds a significant amount of chests, that get replenished whenever the party leaves and comes back. Explore the Serpent Spine mountains north from Britain, and locate the entrance of Dungeon Despise on the north side. There are two rooms on the top floor, and they hold a total of nine chests (including the remains of Mimics). They will grant an average of 450 G per visit.