At least one of the mages guild halls has some grand soul gems in a display case, but I don't remember which one. I also think there are a couple in the Arcane University. If you're high enough level (20 - 30+), grand soul gems will occasionally drop as loot, particularly in necromancer lairs. And no, you can't use a necromancer altar to turn Azura's Star into a reusable black soul gem.
I'm guessing you want black soul gems because humans are much more common (and easier to kill) than monsters with grand souls (but they both create the same level of enchantment). On the other hand, grand soul gems are rare, and converting them to black soul gems is a hassle. Since you have Azura's Star, here's an endless supply of grand souls to use for enchanting the most powerful items: (spoiler:
If you become Grand Champion of the Arena, you can fight monsters weekly. If you're high enough level, around 25+, the monsters will be Minotaur Lords, which have grand souls
).
It doesn't matter how powerful your chameleon spell is when you go to enchant your armor. The level of the enchantment will be based entirely on the size of the soul you use.
Some sigil stones also have a chameleon effect, and at high level they'll be better than what you can enchant even with a grand or black soul gem. Transcendent sigil stone will give you 30% chameleon, so you only need 4 pieces of enchanted armor/jewelry to get over 100%.
