I have stoneflesh - but cannot find anyone who sells higher levels (I am currently at Expert in Alteration) even at the mage school?

There is one mage there who only has one convo option, if I can help with any business, he simply says: any official business, no, no. Is he the alteration mage?

I am level 28 and dying too often with archers - so I need to find the next level spell (I have all the unarmored perks I can have currently) for armor.

College only for the best spells, have you tried them all in the living halls? I got the option for master level restoration when my skill was at 90 odd, assume it's the same with the other schools.

Only the college specialists offer the best spells. Tolfdir is the Alteration specialist, and he's usually in the students' quarters, not the instructors' quarters.