Precision Voltage Reference Module with 5 Channels 1.24V/2.5V/7.5V/5V/10V&5 Levels 10R/100R/1K/10K/100K Resistors of Calibration

Nice fix. You're in luck - they sell fairly precise V and R sources for cheap now. Picked one up to check cal on all my meters. They even do a test on each individual one at the factory and put a sticker on the box with the exact voltages to 4 decimal places, and the temp and humidity at the time. Just make sure you use a burly USB-C power supply for it, it doesn't like cheap 5v only bricks. https://www.amazon.com/dp/B0DM8L151J