Besides buying directly from manufacturers, here is a list of resellers.
AFRICA
Cape film supply S8 / 16mm (South Africa)
AMERICA
Film Photography Project D8 / S8 / 16mm / 35mm (USA)
Toeppen Film Store D8 / 16mm (USA)
Spectra Film and Video R8 / S8 / 16mm (USA)
B&H D8 / S8 / DS8 / 16mm / 35 mm (USA)
EUROPE
Re-voir S8 / 16mm (France)
Super-8 France S8 (France)
Color film archives S8 / 9,5mm (France)
Cine super-8 S8 / D8 / DS8 / 16mm (France)
Wittner Cinetec S8 / S8 / DS8 / 16mm / 35mm (Germany)
Super8shop D8 / S8 / DS8 / 16mm (Germany)
Cine-Mechanics S8 / 16mm (Russia)
The widescreen center : 8 / S8 / 16mm (United Kingdom)
OCEANIA
Nanolab : 8 / S8 / 16mm (Australia)