Extra virgin olive oil is approximately 71.5 BRIX (ranges from 71 to 72, so calibrating to 71.5 puts you close). The refractometer should have a BRIX scale reading as well as % water- that's what's usually used for calibration.
I bought one from Amazon (Ade Advanced Optics Brix Refractometer - 58-90% range) that works pretty consistently, and was less than $30. Make sure you buy one that has the right range for honey.
Dioptric fluid (used for precise calibration) can be expensive, but another trick is to buy a small amount, calibrate the refractometer, then immediately measure a bottle of olive oil. Once you know the exact measurement of that specific bottle of olive oil, you can use it to do calibrations very inexpensively.
Good luck!
(Note: even a decent factory calibrated refractometer still requires calibration in use, because the reading varies with temperature- you need to calibrate it for the temperature of what you are measuring. There should be a small set screw that you turn for calibration that moves the scale reading as needed to zero it in.)