Spectrophotometric Values of Individual Nucleotides
If it's so important to you that you need a quotable source for this, I'd measure it myself with the equipment I use for the real experiment. 

Otherwise I'd suggest you look at a chemical database such as NIST: 
http://webbook.nist.gov/cgi/cbook.cgi?ID=C58617&amp;Mask=400 and use the spectra that can be found there.