Where can this be found? In one game I have that ring plus the ring that gives extra spells level 1-4. I think one or both came from the planar sphere quest (my character was not a mage either). When I couldn't find the ring of wizardry but only found the ring of level 1-4 spells, I thought it might be on the shadow dragon, but it's not there either.

There are two. The first you can get your apprentices to make if you own the planar sphere. The second is on the corpse of one of Galvarey's goons after you go to speak with them in the Harper hold.

I think it is on the Mage in side the harpers hold, but you only meet him in Jaheria harper quest.

Sounds right, though I think it is on the random encounter after you've torn up the hold. Funny, the ring will say it's commissioned by me when it wasn't.

You might be able to pickpocket the ring in the Harper hold, but I've never tried it. Too late this time around, but I'll try next time.