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Well, there are a lot of places, let's see:

15 (maximum) can be found during Janet's quest (just explore the cave before finishing the quest);

4 can be found in the Temple of the Spellbinding (north-east of Rahib), specifically in the cave below (you need dispel to access 2 of them);

2 can be found south of Cornelius Pass (western exit) in a mountain path. near the center of the map, you can see a small pond, and a rocky protrusion with a tree in front of it. You should be able to see something shining behind the tree, on the wall;

2 more can be found during a mage guild quest which sends you to this same area. You'll get to uncover a cave during this quest with an orc living inside, the crystals are there;

3 can be found in the second temple (F1), there is a secret room (you can access it by using push on a misaligned column);

8 can be found in the third temple (B2), there is an invisible path to a secret room (detect magic reveals where) with the crystals;

5 can be found in the fourth temple (B1), use detect magic near the north-west corner of the map (there should be a single lever nearby). A part of the wall will shine, revealing where a secret room is;

There are more, but you get the idea. You can try to CTRL+F for "mana crystal" in the walkthrough. Even if it's more focused on quests, there should be plenty of locations listed (I generally try to list everything you can find in quest-related areas there).