If you want an official answer that isn't Paladin, Hexblade, or Bard, you're SOL. There is no official feature outside of those three options.
Your best unofficial answer is to ask your DM if they'd be willing to let you. A variation on one of the "Initiate/Adept" feats might work, e.g. instead of letting you use a spell once a day without spending a slot, the feat actually adds a single specific spell to your spell list. That plus a +1 to your main casting stat sounds like a perfectly functional feat--you'd only take it if the single spell you get is really worthwhile, and if it is, it's definitely powerful enough to justify a "half-feat."