In Fallout 3, suitcases are usually found in inhabited (or once inhabited) areas, such as: Springvale, Rivet City, and Tenpenny Tower. In Fallout: New Vegas, they are found in many of the settlements like Freeside. Quite a lot of them can be found in the Camp McCarran terminal building.
Suitcases usually hold various pre-War clothing and possibly bobby pins. Frequently, however, they are empty.