Depending on the what level of precision you need for your experiment, you may need to distinguish the grade of copper that you are using to conduct the experiment. Pure, oxygen-free copper are usually used for high vacuum environments, and thermoexpansion curves for those can be found - I found one from UCSD at http://aries.ucsd.edu/LIB/PROPS/PANOS/cu.html. The graph for thermoexpansion (the one at the bottom of the page) is fairly crude. But the data set in the table above is more complete. You can probably plot it out yourself and make a nicer graph.
However, the copper that you are using is probably a copper-nickle alloy, and according to this paper, http://nvlpubs.nist.gov/nistpubs/jres/39/jresv39n5p419_A1b.pdf, you can get very different expansion coefficients for various alloying percentages. (It seems that these differences are on the order of the differences caused by temperature changes)
If you are doing this for a high-school report, you don't have to worry too much, as long as you state in your conclusions that discrepancies may be caused by alloying species. If you are doing these calculations for precise applications, you may want to consider finding out the grade of copper. The graphs for specific grades are usually harder to find (I didn't have any luck after a quick search)