Without buying an expensive calibration tool, how can I accurately calibrate my 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.
Bring them to your university or employer, and see if you can get them to calibrate them for you the next time they do their calibration run.
No, I'm serious. I expect that most schools and employers will be happy to see that their students and employees are developing their skills outside of the school/work environment, and when you've got a production line set up for a hundred meters already, the addition of one (or a few) more meters isn't that big of a deal.