@ Pinante, a 30 second search of the intercape website showed up where to go.
Depart From JOHANNESBURG -
Intercape Office, Park City Transit Centre, Johannesburg Station
To GABORONE - Intercape Office, Kudu Shell Service Station, Queens Road
Hell I even have GPS co-ordinates for you. GPS: 26.197072S 28.041079E
Intercape Bus Stop Information: JOHANNESBURG