I would buy a calibration fluid. They aren't expensive. I believe Randy has a recipe for a calibration fluid. You should calibrate closest to what your testing. Rodi may be close buy will be off some.
My Milwaukee Instruments Seawater Digital Refractometer came with calibration fluid, and was used to calibrate the unit... Just happens I also tried RO/DI water with the unit and it was spot on... If you have "calibration fluid" use it but if you're in a pinch I personally feel 0-TDS RO/DI will also work.