There is a 3rd vendor you unlock in 1-3 as well, who sells you the BIGGEST spells.
Mmm, my preferred method for a mage is to get 30+ Intelligence, 10 Faith, and then equip bow/talisman in one hand and Silver Catalyst in the other, with the Avarice Ring and the ring that gives you an extra Magic Memorization slot, plus the Silver Bracer if you are female.
Memorization list: Soul Thirst/Soulsucker(or Firestorm if you don't have Soulsucker), Evacuate.
Get to Pure Black world tendency in Shrine of Storms, start at the 3rd archway, and run backwards. Bow down the Reaper on the way, and then use Soul Thirst just before the Black Phantom. Walk up to him(if you run you'll aggro him). Use Firestorm or Soulsucker(NG+ only). Run off the cliff to start the process again. When you want to cash out, use Evacuate instead.
This takes me roughly 5 minutes per run, and using Firestorm in normal I get 30K or so per run, using Soulsucker in NG+ I'm getting 92K souls per run.
You can make it go faster by getting Int to 35, which is enough to use all three spells when you have the Silver Catalyst equipped. You can kill the Reaper with other methods, but I prefer the bow due to the Phantom next to the Reaper who can magic beam of death you if you get unlucky.
Note that if you do this, you need to get away from Pure Black World Tendency before you advance to the next level of NG+, because Black Phantom Satsuki is nearly impossible to kill if you don't.