This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Here's a list showing at least one location/situation in which you can
find, buy, or be given a Materia orb. If a disc number is given, then it
means that you can only get it during that particular disc, or that it
isn't available until that disc:
MP Plus - Purchase at Cosmo Canyon/Mideel. HP Plus - Purchase at Cosmo Canyon/Mideel. Speed Plus - Gold Saucer, a Battle Square item. Magic Plus - Bottom of the ladder in the caves beyond the Forgotten Capital (Disc 2). Luck Plus - At the Ancient Temple (Disc 1). EXP. Plus - Purchase at the Gold Saucer's Wonder Square. Gil Plus - Purchase at the Gold Saucer's Wonder Square. Enemy Away - Gold Saucer, win this at the Chocobo Square races. Enemy Plus - Gold Saucer, a Battle Square item. Chocobo Lure - Buy from Choco Billy at the Chocobo Barn. There's another one which appears next to the stable at the Chocobo Barn (Disc 2). Pre-emptive - Gold Saucer, a Battle Square item. Long Range - Found in the Mythril Mine. Mega All - Found in the last dungeon; it's floating in the air and you have to grab it while jumping (Disc 3). Counter Attack - Caves in Mt. Nibel, or as a chocobo race prize. Slash-All - Found near a snapping plant in the Old Forest. Double Cut - Found in the wrecked underwater plane. Protect - In the garden next to Aeris' house. Underwater - Offer the Guide Book with the trader at Kalm. HP<->MP - Cave in the desert peninsula near North Corel. W-Magic - Found in the last dungeon; search the trees north of the shining light in the outside area. W-Summon - Gold Saucer, a Battle Square item (Disc 2). W-Item - The south end of the tunnels beneath Midgar (Disc 2). All - Sector 7 Slums, the Training Hall. Counter - In the last dungeon in the center of the patch of shiny light in the outside area (Disc 3). Magic Counter - Gold Saucer, win this at the Chocobo Square races. MP Turbo - At the Ancient Temple (Disc 1). MP Absorb - Shop at Wutai, during Yuffie's sub-quest. HP Absorb - Chest in the attic above the cat room in Wutai. You must complete Yuffie's sub-quest first. Elemental - Shinra HQ, answer Domino's riddle correctly the first time around. Added Effect - Cosmo Canyon, found in the Gi Tribe's cave. Sneak Attack - Gold Saucer, win this at the Chocobo Square races. Final Attack - Gold Saucer, win if you complete the 'special' battle at the Battle Square (Disc 2). Added Cut - The 'map' area before the snowy field (the one where you can pass out). Take the northeastern path at the mountain crossroads and look for this materia while you're traveling along the repeating path (Disc 2). Steal as well - Extinguish the fires in the cave at the Dachao Statue area east of Wutai town. Quadra Magic - Cave at the end of the peninsula near Mideel. Steal - Find in the sewers prior to fighting Aps (Disc 1). Sense - Midgar, in the playground after Sector 7 is destroyed. Throw - Yuffie comes with one of these. Morph - At the Ancient Temple (Disc 1). Deathblow - Path in the jungle area near Gongaga Town. Manipulate - Cait Sith comes with one of these. Mime - Cave beyond the mountains in the Wutai Area. Enemy Skill - Refer to the 'How do I use Enemy Skills?' question. Master Command - Cosmo Canyon, master the required materia. Fire - Red 13 comes with one of these. Ice - Cloud comes with one of these. Earth - Purchase at Kalm town. Lightning - Cloud comes with one of these. Restore - Near the reactor during Cloud's first mission. Treatment - Purchase from Kalm/Gongaga Village. Revive - Purchase at Junon or Costa Del Sol. Seal - Purchase at Junon. Mystify - Purchase at Cosmo Canyon. Transform - Cait Sith comes with one of these. Exit - Purchase from Rocket Town. Poison - Shinra HQ, chest near a Save Point and Jenova. Gravity - Dropped after beating Gi*Nanataku in the Gi Cave. Barrier - Purchase from Rocket Town. Comet - Found in the Forgotten Capital. Time - Purchase from Gongaga Village or Rocket Town. Destruct - Purchase from Condor Fort (Disc 2), or Mideel. Contain - After Mideel is destroyed, find the hyperactive kid and feed his chocobo a Mimett Greens, then scratch the chocobo's ear (Disc 2). FullCure - Room behind the shop at Cosmo Canyon (Disc 2). Shield - In the lake area of the final dungeon (Disc 3). Ultima - Given to you for free if you save North Corel. If you mess up, it costs 50000 gil. If you didn't even make it to the Shinra train, you can't get it. Master Magic - Cosmo Canyon, master the required materia. Choco/Mog - Listen to the chocobos at the Chocobo Barn (Disc 1). Shiva - Rescue Priscilla from Bottom Swell at Junon (Disc 1). Ifrit - Kill Jenova Birth aboard the cargo boat (Disc 1). Titan - Check the ruined machinery near Gongaga Town. Ramuh - Check the alcoves in the Chocobo racer's room (Disc 1). Odin - Open the safe in the Shinra Mansion at Nibelheim. Leviathan - Win from Godo at the Pagoda of Five Strong in Wutai. Bahamut - Ancient Temple, after fighting the red dragon boss (Disc 1). Kjata - In the forest past Bone Village; it's floating around. Alexander - Refer to the 'Woman who tells me to go away' question. Phoenix - Win the Huge Materia Simulation Battle at Fort Condor during Disc 2. Neo Bahamut - Found in the Tornado Maze, on the same screen with the Save Point, near the bottom ledge (Disc 2). Hades - Found near the ruined helicopter in the wrecked underwater plane (Disc 2). Typoon - Found in a large item bag in the Old Forest (Disc 2). Bahamut ZERO - Cosmo Canyon, take Bahamut and Neo Bahamut there. Knights of Round - Use a Gold Chocobo to reach Round Island (Disc 2). Master Summon - Cosmo Canyon, master the required materia.