As a necromancer, it's very important to not just have undead, but to have control of those undead. Quality over quantity. Thus, it's helpful to ensure you can maximize your daily castings of Animate Dead. As a 3rd level spell, it's relatively easy to recover castings of the spell; conversely Create Undead is a 6th level spell and thus much more difficult to gain more castings on, I could not locate any practical magic items that would permit you to reliably get multiple daily castings of Create Undead.
- Ring of Spell Storing is a good way to give yourself an extra daily casting of Animate Dead. As a Sorcerer, there's a good chance that you'll have a means to ensure you've an extra 3rd level slot daily that you can pour into it to power that spell.
- Reserve Ioun Stone is another good way to get in an additional Animate Dead in daily.
- Pearl of Power is the last way I've identified to regain another spell slot and with it another casting of Animate Dead.
As for the undead themselves, the list of ways to enhance them is far too numerous to detail here as any armor, weapon, etc. you can find you could slap on them and have them be 'buffed'. That said, the disposable nature of undead does open up the possibility of spending that Bonus Action in a more interesting manner other than 'mercilessly kill all my hated enemies'.
- Shove a Portable Hole into a Bag of Holding right next to my hated enemies.
- Break a fully charged Staff of Power/Staff of the Magi right next to my hated enemies.
- Get next to my hated enemies and throw Dust of Sneezing and Choking into the air.
- Spray this Decanter of Endless Water at my hated enemies.
- Wear this belt of Storm Giant Strength, smear yourself in Sovereign Glue, and grapple my hated enemies.