You should try the magic shop at magic arx. The items there refresh every while and sometimes good charms show up.

There are traders when you are in the endgame that sell them. Not saying that other traders don't but I only found them before that.
I'm now level 45 and only have 4 very large charms so don't wait for them.
The only very large charms that are significantly better are the resistance, +40 vs +20 for large.

Charms are fairly random drops and not too many good ones in the shops. I found a wondering witch above the Orc camp that was selling some nice charms. I almost didn't trade with her since she looked like a poor NPC.
Only when I reached the Wastelands did I start to find some nice charms and the Orc was selling some good, large charms as well. You may ask yourself, "What good are the large charms now?" . . . =)
Edit: Corrected Land of the Dead to Wastelands

Thanks a lot guys. Very helpful.

Update for this question . . .
**Spoiler**
In Verdistis, after Mpenzak's wagon is burned . . . he will stand just outside the Ducal Inn. In my current game, he offered two large (silver) charms, one for a large Intelligence bonus and one for a large Vitality Bonus . . . both were 35,000 gold pieces each. These were the best charms I have found at this stage in the game . . . from a merchant . . . I have found a couple during battle as well.

The only one I consistantly found selling very large charms was the orc in the land of the dead, but everytime you came back to him he would have 2-3 very large ones stocked.

IIRC, the last stage of the game doesn't play out in the Land of the Dead, but in the destroyed Orcish territories. The character is fully alive and well.
That said, Imps there also have some chance of dropping very large charms.

I don't know of anyone who consistently sells very large charms. My advice is just to try to barter with everyone you meet and hope you get lucky. I've found VLCs in the hands of the Arx magic shop, a general store somewhere, and in the possession of a few random dwarves. I find spellbooks the same way.

Kroxy. But he is near the end of the game in the village in the wastelands.
Don't you mean ars magicana?

You are correct . . . belated thanks . . . =)
Seems I can type much faster than I can think and both are subject to errors . . . ;)