

If you wait until she gets up from her chair and walks over to the counter, you can usually steal the soul gems without being seen by crouching behind her until you're hidden.

Those 3 to 5 in the Enchanting shop are probably your best bet, and the NPC won't attack you if she sees you stealing them, she'll just give you a bounty. The soul gems will provide more recharging Magicka.I think the OP wants to farm them without too much effort. The enchantments newly made will be 20% stronger from learning each skill mentioned below.

You can also recharge your weapons using soul gems. You can find a soul gem in the Whiterun Castle in Farengar Secret-Fire’s room near the throne. The Human souls which are equivalent to grand souls can only be captured in black gems. Each holds souls proportional to its described room. Petty, Lesser, Common, Great and Grand these are the soul gem sizes.
