You can get 10 Master Spheres from Creating Nemesis.
In order to get the 10 Master Spheres you need to capture ten of each of the fiends roaming all the areas as follows.
Besaid, (Dingo, Condor, Water Flan), Kilika, (Dinonix, Killer Bee, Yellow Element, Ragora), Mi'ihen Highroad, (Mi'ihen Fang, Ipiria, Floating Eye, White Element, Raldo, Vouivre, Bomb, Dual Horn), Mushroom Rock Road, (Raptor, Gandarewa, Thuder Flan, Red Element, Lamashtu, Funguar, Garuda), Djose Road, (Garm, Simurgh, Bite Bug, Snow Flan, Bunyip, Basillisk), Thunder Plains, (Melusine, Aerouge, Buer, Gold Element, Kusariqqu, Larva, Iron Giant, Qactuar), Macalania, (Snow Wolf, Iguion, Wasp, Evil Eye, Ice Flan, Blue Element, Murussu, Mardet, Xiphos, Chimera), Bikanel, (Sand Wolf, Alcyone, Mushussu, Zu, Sand Worm, Cactuar), Calm Lands, (Skoll, Nebiros, Flame Flan, Shred, Anacondaur, Orge, Coeurl, Chimera Brain, Mal boro), Sunken Cave, (Yowie, Imp, Dark Element, Nidhogg, Thorn, Valaha, Epaaj, Ghost, Tonberry), Mt. Gagazet, (Bandersnatch, Ahriman, Dark Flan, Grenade, Grat, Grendel, Bashura, Mandragora, Behemoth, Splasher, Achelous, Maelspike), Inside Sin, (Exoray, Wraith, Gemini, Demonolith, Great Malboro, Barbatos, Adamantoise, Behemoth King), Omega Dungeon, (Zaurus, Floating Death, Black Element Halma Puroboros, Spirit, Machea, Master Coeurl, Master Tonberry Varuna).
Some of these mosters might be located in two places or more at once. You'll need their capture weapons, only sold at the Battle Arena.
You allso acquire 2 Master Spheres by defeating the Dark Aeons. But you hardly receive this as your reward. That only happens with a Overkill. Usually you get 2 Dark Matter. This is allso good to have as well.