There are numerous places to get online, mostly charging between B$3 and B$4 per hour. Some cafes and restaurants have wi-fi, as do most accommodations.
The Instituto Geográfico Militar sells topographic sheets of all areas of Potosí department.
ATMs are common in the center of town. Lots of businesses along Bolívar, Sucre and in the market change US dollars at reasonable rates; stalls along Héroes del Chaco also change euros and Chilean and Argentine pesos. Many hotels now accept credit cards, primarily Visa and Mastercard.
InfoTur de Potosí has quite helpful Spanish-speaking staff, good city maps and loads of brochures on city sights; it's located at the entrance to the Torre de la Compañía de Jesús. Another useful site is www.brujulaturistica.com/potosi.
The tourist police on the ground floor of the Gobernación building can be helpful; there's also a branch at the new bus terminal.
Head to the Migración office for visa extensions.