I have no pointers on sources for donors or cores. However, in answer to Pat's question regarding capillary tube repair/replacement, it is possible and with some improvising and soldering skill you can do this yourself. The dry ice that Peter mentioned is great and some grocery stores (around here at least) will sell it to you. Failing that, the type of salt/ice bath used for homemade ice cream will work. However, monitor the temperature in the ice bath and don't start your repairs until the temperature is close to 0 oF
The repair method is described here:
https://www.ply33.com/Repair/tempgauge
I have successfully repaired three gauges using this method. A fourth one I attempted to fix was a complete waste of time and I never should have attempted it. As a warning about the repairs... look at the gauge you want to repair. If it died because of capillary tube failure the needle will be sitting on its low temperature stop peg. If the gauge died for other reasons (like extreme overheating) the needle will be "stuck" somewhere off the low end. A gauge that is NOT sitting on its low end may (and likely does) have other issues requiring a lot of tinkering to calibrate correctly. I wouldn't bother.
The other warning is to prepare the tube ends carefully before soldering and minimize the time and amount of solder you apply. It's VERY easy to get so much solder on the splice tube that it wicks into the capillary tube and plugs it.