[Archonsod]

As far as I know most sites are level 2, with one or two per school which are level 3.

Whether it's worth casting spells depends on the site frequency set at the start of the game. A high frequency means there's a good chance of at least one site per province, however with a L2 mage you'll likely find the one's of interest. In this case, you're better off casting something like Acashic Record to find the sites you otherwise couldn't (i.e. those of a school you can't access).

Location also seems to play a part. Deserts and Mountains tend to be good for fire sites, forests for nature, coasts & rivers for water and so on.