Nyxil's solution has all the basics, but I'm just adding some information from my personal experience for the more difficult to obtain miracles.
I bought Bountiful Sunlight and Wrath of the Gods from Chancellor Wellager in Drangelic castle in NG++ (playthrough 3). I just blitzed through the four primal bosses to get to the Chancellor ASAP. you can also get them by ranking up in the Blue Sentinels covenant but that requires many, many successful invasions (500, I believe).
I got Sunlight Spear by doing white soapstone boss assists while in the Heirs of the Sun covenant to gather the necessary 30 Sunlight Medals. My boss of choice was the Looking Glass Knight as he's pretty easy and was a very active summoning spot (at the time... YMMV).
Blinding Bolt can traded from Straid in the Lost Bastille. It requires the Old Kings soul from the Old Iron King in NG+ (playthrough 2).
You can get Soothing Sunlight by invading and Killing Licia of Lindeldt. When you get near her (later in the game, I believe) after she relocates to the adjustable path contraption between Heide's Tower and Huntsman's Copse, the Crushed Eye Orb gives you a notification and you can use it to invade her.
EDIT: The Crushed Eye Orb is found in a chest in the Undead Crypt. When you pass the NPC that tells you not to use light, you'll enter an open area with two knight enemies on a bridge on the right. Drop off to the left onto a lower ledge, follow it briefly, and then climb a ladder to a higher platform with a chest that contains the Crushed Eye Orb.
Everything else can be bought from a vendor (there are several that sell miracles) or traded from Straid with a boss soul.