Specifically i'm trying to get the thunder spell, where is that?
Viewing post in Caliross, The Shapeshifter's Legacy comments
If you mean the "Lightning Storm" spell, you get the book as a reward for one of the mages guild quests. If you are a member already, talk to the lady sitting at the library (same room as Alpharius) and she'll offer you the quest (Under The Moonlight).
You can find more details about the quest in the walkthrough,
but do let me know if something has you stumped.