If you can't see where they're coming from, put some fine flour mixed with sugar near where you see them. They will track the flour and you can see their trail. For killing them, I used to do the boiling water outside, and use vinegar mixed with water for inside. Both kill them outright. I switched though, and now once I see where they are coming in I put cheap little ant traps there. They grab pieces from inside the trap and take them back to the nest, where it kills the whole colony. Also considering a better seal where the ants come in. Caulking can work wonders if they're coming in at the edge of the floor, a window, etc. If they're coming up through larger holes drilled for plumbing or electric, seal those with foam insulation and see how that works.
