Ultrastable capacitor wrt temperature
Andeen Hagerling make ultra-stable capacitance standards up to about 100pF, part number AH11A.  They are stable to 0.001ppm/C, and also have low drift (0.1ppm/yr), high Q, and low voltage dependence.  They are probably too expensive for your application though.  They achieve this kind of stability with fused silica capacitors in a temperature controlled oven.

If you want a precision distance measurement cheaper than a laser system, you might consider a capacitance sensor. But you're going to have a rough time beating the price and performance of the laser system.