Sumba is harder to reach than most Indonesian islands, which is precisely why it has remained the way it is.
By air: The quickest route is to fly. Waingapu in the east has Umbu Mehang Kunda Airport (WGP); Waikabubak in the west is served by Tambolaka Airport (TMC). Both have daily connections to Bali (Denpasar) and Kupang on Timor, with some services routing through Lombok. Citilink, Wings Air, and Garuda operate these routes. Flights are inexpensive by international standards but seats fill quickly around festival time.
By ferry: PELNI operates passenger ferries connecting Sumba with Bali, Lombok, Flores (Ende, Labuan Bajo), and Kupang. Crossings are long — Bali to Waingapu is roughly 40 hours — but cheap and atmospheric. Schedules are irregular; check the PELNI website or local agents in Bali for current departure times.
The ferry port for Waingapu is in the city itself. Waikabubak is served by the port of Waikelo, about 45 minutes north by road.