I realised I can only find novice spells on vendors.

Is that because I don't have any apprentice perk on any school of magic yet? Or will I find them eventually but not be able to use them till I get the perk?

No, those perks only make them use less magic, you can still use any spell without them.

once your skill in any magic reach 25, 50, 75 and 100 the spell tome will become avabile you can find them in dungons at any level but to buy them you skill level has to be equal to the spell level e.g destruction lv of 50 will mean to can buy fireball, blizzard and chain lighting

tho i think any tomes higher than apprentice can only be found on the mages collage

So why the vendors don't have any spells above novice? Is it because I'm still level 7 and they will appear when I'm a higher level?

use the spells you already have until you level up a little and they will start selling them

OK then, thx for the answers

Its the famous LVL SCALING in action.
They could have made restriction only, but now they had to add LVL scaling.

Yeah it's mostly based on finding the Magi who knows the magic books, and whether or not your skill level is high enough that you can reasonably perform it (to buy/learn permanently). Perks are just personal for your character.

Although the strangest thing happened to me today. SPOILER BLEAK FALLS BARROW. I was running through the first dungeon (Ancient Nord crypt), and I somehow managed to get like 6 Draugr on my tail. I was fleeing http://www.youtube.com/watch?v=PUkRatOSxaI (stopping of course to open any chests I see being a Mage/Scoundrel focusing my points in Pickpocket and Lockpick). I found a scroll of Guardian Circle in one chest, and a scroll of Fire Storm in the next. In an epic moment, I cast the Guardian Circle to send the bones fleeing, and then as they gathered I obliterated them with the Fire Storm XD