With 40 hours of gameplay i've seen like 2 tomes drop from 'random' and they were both novice/apprentice. You're not understanding my question.
.... Yes, I am understanding your question. You're not understanding how my (or anyone elses) posts relate to your question.
Your best bet is going to be spell tomes, if no one is selling them to you. 2 drops is a very small number. From memory, from Oblivion, most dungeons had very low drop rates for spell tomes. Mages were more likely to have them.
Secondly, your question was answered in the very first post.
To clarify:
The higher levels of magic become available from merchants once you level your skills up high enough.
I.e. Adept level Conjuration skills become available when Conjuration is at 50.
You can only learn an adept destruction spell from a tome or a merchant when your destruction is at 50.