So far I have been using the Lion's Mage Armor set. It is a really good set that increases casting speed. It's found in the Shaded Woods area right before you fight the boss.There is a really good hood too called the Hexer's Hood which adds one to your spell usage. I forget where I got it from it was really early on though.
The rings depends on what you really what, for my caster I have got: ring of restoration, second dragon ring +2, clear bluestone ring +2 (best ring in the game for caster in my opinion), and southern ritual band +2.
Here is a link to what these rings do and where to find them.