Where can I send my 0.005% 1 PPM tempco calibration resistors to find out their "true" value?
Calibrate the instrument

Modern DMM's use internal tables of data to implement multi-point calibration. You will not be able to do this yourself without a service manual and their test/calibration software suite. 

Manual calibration with resistors will allow you create your own calibration function, but then you will have to manually apply it to each measurement (or suck the measurements through a data drain and do it there). 

Factory calibration is not expensive and you don't need to be a huge enterprise to access it.

There are also 3rd party calibration services and labs. Most test equipment rental companies also have attached labs. 

Here is an example of US pricing for reference. It is about 10% of original purchase price.