Helgen is pretty good, but it takes too long to reset. If I had like four or five lairs that reset in the shortest number of game days, I could round-robin through them anytime I need a couple of charges.
I'm particularly interested in mage lairs that reset their supply of soul gems. I keep going back to Fellglow Keep, but so far nada. Probably because there's a quest connected to it I haven't started yet.
For RP purposes, I currently don't kill anyone that would earn me a bounty. Stormcloak, Imperials, Silver Hand and orc encampments are also off-limits for now. I know, that cuts my supply by more than half, but there's only so much kill everything that moves I can do before getting bored. Gotta have some limits.

. Come to think of it Valtheim Towers seemingly respawned every other day earlier in my game for a little while, then stopped completely.