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.
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.