Are you looking for fresh, dried, canned or a restaurant that serves them? I cook the dried ones at home. I usually find them in the "Spanish" foods aisle. labelled as Lima Grandes (or Gigantes). Mama Tina's is my favorite brand.
If she's looking for southern butter beans, they are smaller than the gigantes. Butter beans are really the same as lima beans. Maybe a bit smaller. I would imagine that a green grocer might sell them aleady shelled. I'd ask at your green grocer and your farmer/s market.