Take the Neriak Stone in PoK to Corathus Creep (west side in Neriak) head West once in Corathus to Undershore zone (I used lev/invis for grins).
Once in Undershore stay on the upper path and follow it to the left across a few bridges. You will come to a few small huts, the vendors name is Brother Hidden Path and he is behind the first hut at loc negative 910 positive 732.
Spells cost between 16 and 23pp.
Have fun!