There are 23 Miracles in the game. I will first list them in the order they appear in your inventory, then I'll explain how they can be obtained.
*** Spoiler - click to reveal ***Heal
Great Heal Excerpt
Great Heal
Soothing Sunlight
Replenishment
Bountiful Sunlight
Force
Wrath of the Gods
Emit Force
Magic Barrier
Great Magic Barrier
Homeward
Seek Guidance
Karmic Justice
Vow of Silence
Tranquil Walk of Peace
Lightning Spear
Great Lightning Spear
Sunlight Spear
Sunlight Blade
Darkmoon Blade
Gravelord Sword Dance
Gravelord Greatsword Dance
Nine can be purchased from NPCSSeek Guidance: sold by Rhea for 500 souls
Heal: sold by Rhea for 1000 souls
Homeward: sold by Rhea for 1000 souls
Force: sold by Rhea for 1000 souls
Great Heal Excerpt: sold by Rhea for 2000 souls
Magic Barrier: sold by Rhea for 6000 souls
Wrath of the Gods: sold by Rhea for 10000 souls
Great Heal: sold by Rhea for 1000 souls
Note that some of these miracles are sold by Petrus in Firelink Shrine but all eight can be bought from Rhea.
Karmic Justice: sold by Oswald for 40,000 souls
Six are found on a corpse or dropped by a specific enemyBountiful Sunlight: 100% drop by an enemy in the prison section of the Duke's Archives
Soothing Sunlight: 100% drop by an enemy in the prison section of the Duke's Archives
Great Magic Barrier: found on a corpse in Ash Lake
Sunlight Blade: found in a chest after defeating Dark Sun Gwyndolin
Tranquil Walk of Peace: found on a corpse in the Catacombs
Vow of Silence: found on a corpse in the annex area of the Painted World of Ariamis
Six are from covenant activity (joining or ranking up)Gravelord Sword Dance: join the Gravelord Servant covenant
Gravelord Greatsword Dance: offer 10
Eye of Death to the Gravelord Servant covenant
Darkmoon Blade: offer 10
Souvenir of Reprisal to the Darkmoon Blade covenant
Lightning Spear: Join the Warrior of Sunlight covenant
Great Lightning Spear: offer 10
Sunlight Medals to the Warrior of Sunlight covenant
Sunlight Spear: offer the Soul of Gwyn to the Warrior of Sunlight covenant
Two are from NPC questlinesEmit Force is given by Siegmeyer once he relocates to Firelink Shrine
Replenishment is given by Rhea after defeating Nico and Vince in the Tomb of the Giants
NPC InformationOswald of Carim appears at the bottom of the tower where you ring the first Bell of Awakening.
Rhea of Thorolund is found in the Tomb of the Giants close to where you fall when Patches kicks you off a ledge. Kill the two nearby hollows (her original guards), then talk to her and she will relocate to the Undead Parish and offer to sell you her miracles.
Siegmeyer of Catarina is first encountered outside Sen's fortress. After ringing both Bells of Awakening, he moves inside the fortress near a ramp with a perpetually rolling boulder and bemoans his inability to progress. Use a contraption to redirect the boulders and he will relocate to Anor Londo outside a room containing three Silver Knights. Defeat them and talk to him so he will relocate to Firelink Shrine.
As always, refer to
http://darksouls.wikidot.com/ for specific information relating to items or locations.
This page gives specific information for each miracle.