The long-established Potosí operator, with the head office on Calle La Paz near the Plaza España and a second on Avenida Potosí in Uyuni — the two ends of the trip most people do, since Potosí and the salt flat are four hours apart and almost everybody links them.
Potosí is the point: the Cerro Rico above the city was the richest silver mountain in the world and paid for the Spanish empire, and men still work it in cooperatives under conditions that have not modernised much. Going in is not a theme park and should not be treated as one. The right question to ask an agency is what the guide's relationship with the cooperative is, and whether your entrance fee reaches the miners.
They run a separate reservations line for agencies, which tells you a good part of their business is wholesale.