Hi Lost sheep!
It's actually a very good question!! So good, that I wish I "knew" the answer!
I don't recall this being offered at any of the guilds for training (but it has been a while since I have played). It would seem logical that you'd find training, if it were available in the Magic Shoppe/Wizards Guild. If it isn't offered, you can only up it by putting points into it.
What level have you reached in Sorcery??
Wyv