Thanks for the help, all. Will look into Orphan's Rock and the Windhelm Market. Windhelm is one of the cities I've deliberately been avoiding (to make sure all its follower candidates are locked-in at a high level when I make my first visit).
From the Wiki it seems to imply there's one at Serpent's Bluff Redoubt - is that one outdoors or indoors?
Do you use an extremelly outdated PC or a PS3 (not trying to say, that 360 is better, but Skyrim seems poorly optimized for PS3)
PS3. (That's also my response to people sugesting command console commands to solve this problem).
Try doing a kill-kill-enchant-enchant cycle instead?
Only one Black Star. Black soul gems aren't plentiful enough to have a substantial accumulation of them. When I want to do 10 consecutive max enchants, that's usually 10 emptyings of humanoid souls from my Black Star, which usually means ten distinct trips to the enchanter. (Though one time I did have a lot of success killing Necromancers in a ruin that had an Arcane Enchanter, so I was able to make several enchants from my Black Star without reloads).