Vernier does not sell calibrating solutions for the ORP Sensor.
ORP reference solutions can be purchased from Hach Company. The order code for a bottle of 500 mL of 200 mV reference solution is 25M2A1001-115. The order code for a bottle of 500 mL of 600 mV reference solution is 25M2A1002-115.
Cole Parmer also carries ORP standard solution, order code: EW-05730-55.