Many Tosibox products and accessories use SIM cards.  Please use this reference to determine the correct SIM for your device:

Credit: Justin Ormont via Wikimedia Link

Tosibox Lock 500i Series - Micro SIM (3FF)

Tosibox Modem TB4GM2 - Mini SIM (2FF)

MultiTech MTC Series - Mini SIM (2FF)

MultiTech MTR Series (MTR-@@@7) - Micro SIM (3FF)

MultiTech MTR Series (MTR-@@@1, MTR-@@@2) - Mini SIM (2FF) 

Huawei MS Series - Mini SIM (2FF)

The SIM should be inserted into the Lock 500i with the metal contacts facing the green circuit board, and with the notched corner facing down, as shown:

Please see here to obtain SIMS in North America: