Miracles can either be purchased from certain NPCs, looted around the world or given as a reward by ranking up certain covenants. Since one of them require the soul of the game's last boss you'll only be able to unlock this trophy early in New Game+.
Below is a list of where to get each miracle and how to get the NPCs to sell them to you. Big thanks to Blood_Runs_Cold for compiling most of the information in here!
If you want, you can use the printable checklist from HERE made by Birr1978 to help you keep track of what you have.