How to calibrate multimeters?
You can buy a few 0.1% resistors to calculate resistance ranges cheaply. 

Voltage is tricker - if you have access to several meters you can 'calibrate by consensus', as it is improbable that they will all go out in the same direction. 

Another option is buy a precision voltage reference IC - e.g. AD581 is 10V with 0.1% accuracy.

Current can also be done using voltage across a known, accurate resistance.