You get divination cards randomly from killing enemies. Each area has specific divination cards that drop.
If you hover over the card and look in the middle there is a number of how many you have and how many you need.
One of the easiest to get would be The Catalyst. You need 3 cards and you get them at Vaal City, temple of decay, and temple of decay 2. The door to get into temple of decay will be right in front of you when you fast travel to Vaal City.
Here's a list of all divination cards
https://pathofexile.gamepedia.com/Divination_card. If you want to figure out where your card drops just click on it and it will bring you to a page with all the locations.
After you get a full deck go to highgate and turn them into tasuni for your reward and achievement.