What device can I use as a load to calibrate a power meter accurately?
Electric kettle, clothes iron, rice cooker, toaster, etc., are all resistive loads but will suffer rising resistance with temperature so you need to let them stabilise. You can then measure voltage and current with a calibrated multimeter and calculate the power for calibration. This may be enough to get you within 5% to 10%. Calibration is expensive because precision references are expensive.
Don't forget that if you are using a current transformer (CT) that you can use multiple turns and a small load to do the calibration.