/diy/ - Do It Yourself

>>2717791
Nothing cheap. Professional calibration centers use hermetically-sealed, thermally-insulated machines with built-in ovens that keep the parts at a constant temperature to achieve .001% tolerances and <2ppm/year drift.
If you just want to verify that your machine is good (say, you bought it used), you can get some (relatively) cheap 0.1%/0.01% components, one for each range for each mode, off Digikey or Mouser.