They're in various caves really, and at certain merchants. I know that at least one merchant in Mora Sul will sell a stone tablet or two. Other than that, just look for books called "Bookshelf" instead of "Lectern". Those labelled Bookshelf will either give you +5 alchemy or +5 ancient knowledge. There are quite a few of those scattered around in the desert.
Also you can find pillars, big stone tablets that you can't pick up, but stop to read on. These will also give +5 ancient knowledge from time to time. These are also located all over the desert, especially in the old ruins to the south and south west.
All in all, you should be able to get 100-150 ancient knowledge through the game without any LPs spent in it. As for alchemy - roughly 60-80 is normal for me, without spending points in it, so you can make any potion in the game without ever boosting alchemy with LPs.