I need a lot of soul gems for my Nord who wants to get into enchanting. I want petty/lesser/common soul gems for training, so that's what I'll need for right now. I've been using the enchanting table in the basemant in the Sleeping Giant Inn to enchant weapons and cheap armor I get from Alvor and sell the enchanted gear to the Riverwood Trader and buy his soul gems. It's a nice little loop. But, Lucan(?) doesn't always have a good number of gems. I also keep the hidden chests at Dawnstar, Markarth. and Solitude empty.

Where are more soul gems?

I've always had good luck raiding Dwarven ruins. The automatons in the ruins frequently have soul gems powering them. Sometimes the soul gems have souls, sometimes they're empty.