reference source for copper conductivity vs. temperature?
Data from NIST (table 2)
At low temperatures the resistivity of copper approaches a "residual" resistivity. However, this isn't really a concern until below ~100K.
At higher temperatures the resistivity is best approximated with a linear increase vs. temperature (as you've already found). I plotted the resitivity vs. temperature, and I wouldn't be too worried about non-linearity too much until ~800K.
The Bloch-Grüneisen Formula gives you a function of resistivity vs. temperature.
Debye temperature Θ is a parameter of the function.
Its value for pure Copper is ΘCu = 343.5 K
If you can't find the value of the Debye temperature Θalloy of your particular alloy, you can make some measuremnts over the interesting temperature range and determine it by least-square fitting.