Cook macaroni according to package directions. Meanwhile, cut a thin slice off the top of each tomato. Scoop out pulp, leaving a 1/2-in. shell. Invert tomatoes onto paper towels to drain.
Drain macaroni and rinse in cold water. In a bowl, combine the macaroni, ham, cheese and celery.
In a small bowl, combine the mayonnaise, sour cream, onion powder and pepper. Pour over salad and stir to coat. Spoon into tomato cups. Serve on a lettuce-lined plate if desired with any additional salad on the side.