Well, magic barrier lvl 5 gives you 25 pts/resistance.
Boosting up your res. via purple potions on a temporary basis is another possibility.
Charms can be bought from the mage Kistandalius in Ars Magicana, Corinna, the summoner/Verdistis and orcs drop them, too. But as you said: They're random, there is NO fix place to get them. The fortune-teller you meet once has charms.
In the last part of the game you can buy gold charms from Kroxy, they are the highest ones.
Kiya
BTW, welcome to the forum <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />